Summon Greater Demon has material component "a vial of blood from a humanoid killed within the past 24 hours". I'm having trouble parsing whether "vial" is an actual vial or just a measure of amount? So if I've killed a guy, could I use that person's blood to cast Summon Greater Demon without a vial or do I specifically need to put his blood into a vial before the summoning can take place? Assume for a second that focus is not an option.
A vial is (probably) not needed but is a good idea
It's a fair question
On the one hand, "a vial of blood" could mean you need a literal vial. On the other hand, perhaps you need a 'vial's worth of blood', since a vial is a unit of measurement. A vial is, in fact, on the PHB list of Container Capacity as officially holding four ounces of liquid. So it is a fair question as to whether you need specifically 'blood inside a vial', or whether 'exactly four ounces of blood in any sort of container' will do. Given the ambiguity of meaning in 'a vial of blood', we won't be able to settle this by looking at the spell description itself, but we can look at other spells and see whether any patterns emerge.
Spells very rarely specify exact measures
A lot of spells have sympathetic magic material components in which a small amount of a symbolic substance is used. For the dozens of spells that measure their components in "pinches", "bits", "pieces" (or "small pieces"), "handfuls", and "drops", I was able to find only one that unambiguously calls for a specific measured amount. Clone requires "at least one cubic inch of flesh". It would thus be very unusual for Summon Greater Demon to require exactly four ounces of blood. However, there are a number of other spells that are similar to Summon Greater Demon in that they specify a liquid substance in a container that could be literal or could be a measurement. "A vial" is also used in Commune, Summon Lesser Demons, Rime's Binding Ice, and Create Magen (and "a cup" is used in Armor of Agathys). We can't do anything but circular reasoning here; either they all indicate a specific measurement, or they all denote a specific container type, but without knowing which we can't infer anything about the spell in question.
Spells sometimes call for specific liquid component containers
Some spells do actually specify that their liquid material component needs a specific container, not just a certain amount. Hypnotic pattern, for example, requires "a crystal vial filled with phosphorescent material". By saying 'a vial filled with material', we know that the vial itself, not just the measurement, is essential. Similarly, Summon Elemental requires "water inside a gold-inlaid vial", and Create Undead requires "one clay pot filled with grave dirt [and] one clay pot filled with brackish water...for each corpse" (and note that by definition, an iron pot holds 1 gallon - but we don't know how much a clay pot holds). Further, in each of these cases the material from which the container is made is specified (crystal, gold-inlayed, clay), while in Summon Greater Demons it is not.
A note on material components
We should remember also the rule that material components may be completely substituted by an component pouch or an arcane focus so long as the components don't have a specified cost. Thus for Summon Elemental the vial itself really is an integral, noninterchangeable part of the spell, whereas for Summon Greater Demon both the vial and the blood can be substituted by other things. It is reasonable to assume that a component pouch contains spare vials, less so that it has a constant supply of fresh humanoid blood, while a wooden wand has neither. However, as Adeptus points out, the actual blood is necessary to draw the protective circle, which is an optional part of the casting.
A vial is likely not required, but preferred
We know that if it was important to specify the container as part of the spell, this could be done. As in Hypnotic Pattern, not only could the spell say 'blood inside a vial' (rather than 'a vial of blood'), but it could tell us what material the vial would need to made from. We know that simply saying 'a vial of blood worth 1gp' would mean the component could not be substituted by a focus or pouch. We also know that if it was important to specify the exact amount of blood to be used, this could be done as well, as it is in the Clone spell. Since none of these details are specified, Summon Greater Demons appears to be like the vast majority of spells, whose material components are no more detailed than those of Insect Plague: "a few grains of sugar, some kernels of grain, and a smear of fat." In this spirit, it seems likely that all you need for the spell, absent a focus, is around four ounces of blood in some sort of container.
However, even if you have a focus to take the place of the material components, in this case casting with the components may be strongly preferred so that you can obtain the full benefit of the optional protective circle: "As part of casting the spell, you can form a circle on the ground with the blood used as a material component." The spell may be telling us that four ounces of blood is the minimum needed to make a circle around "your space", such that with less blood you might be able to complete the spell, but not the circle. Similarly, a narrow-mouthed vial, with which one can carefully control the pour rate of the blood, is the ideal container to efficiently trace the circle - were the blood held in a container like a bowl, you might need much more to complete the circle without leaving any gaps.
A vial is a container, not a measurement
RAW you need the container, not just the blood. The spell says “vial of blood” - it means both the vial and the blood.
However, since the “vial of blood from a humanoid killed within the past 24 hours” has no cost, RAW, there is always one in your Component Pouch. How does it get there? RAW doesn’t say.
Equally, RAW, it can be replaced with a focus.
I am going to drink a glass full of water
Despite that, I am not going to melt the glass into a molten pile and consume it as well. I'm just going to drink the contents.
In English, referring to "a container of substance" as short hand for needing "the substance contained within the container" is standard, colloquial usage. It is ambiguous, because language is ambiguous in most uses, but ambiguity doesn't make either reading invalid.
Read it either way
If you make a habit out of not having a component pouch and casting this spell, talk to your DM about "going to the source" for it.
If it is a one-off situation, talk to your DM when doing it. It is possible that the PC doesn't even know, as every time they cast this spell (and the spell's instructions) tells them to have it in a vial!
While it is perfectly reasonable to exclude "blood of a humanoid killed within the last 24 hours" as being a standard ingredient of a component pouch (I mean, maybe when you bought it it comes with that, but it seems extreme to include a delivery service that keeps it fresh and up to date), the vial itself has no cost and seems like something you might have in the pouch.
So even if focus/blood itself is ruled out, saying that the component pouch has a vial you can quickly fill is plausible and reasonable.
A strict reading implies you need a vial full of blood.
The list of items in Other Adventuring Gear section in the Basic Rules includes also a vial, with the relative cost. Hence, this implies that the container is actually one of the material components needed for the casting, the other being the blood from the killed humanoid.
Moreover, the "vial" entry in the Container Capacity section depicts the capacity of it, it is not a unit of measurement. It provides anyway information on how much blood you need for the summoning.
The DM may adapt the rules to the case at hand.
The DM may rule that you can collect the blood in a different container than a vial (a bottle, a flask, whatever may contain a liquid), taking into account that the required volume of blood is 4 liquid ounces. They may modify the quantity of required blood, too.
Moreover, they may rule that you don't need any container at all: for example, in the middle of a battle, your party kills an (humanoid) enemy, then you cast Summon Greater Demon and the blood streams out from the enemy corpse creating a crimson portal from which a Barglura comes out.
A really strict reading of the rules suggests that you have to be versatile in crafting new items...
A direct reading of the component requirements tells us that you need a vial made of blood.