The rules as written are ambiguous
(This is a long answer!)
That's part and parcel of TTRPGs, and especially so for WoD. We can look at the question from a couple of angles, but the rules are ambiguous because they don't explicitly answer your precise question. That means that we can choose between
- Specific interpretations of the rules as they are written
- Looking more broadly at how the rules function in the overall game
- Determining what end results are desirable and appropriate for your
game at your table, and then impose a ruling which supports those
results (whether or not that is unquestionably the correct ruling
1. Interpretations of what's written
I believe that the most faithful interpretation to what's written is that the benefits of Bliss do not stack across different Virtues. It's pretty pedantic, but RAW often is! My reasoning is based around a minor phrasing detail which suggests that at a time refers to per night rather than per use of Bliss. The relevant section of text:
Within one scene after the trance ends, the player
makes an Intelligence + Empathy roll. If the roll is
successful, the vampire may temporarily increase her
Self-Control, Conscience, or Courage by a single dot,
up to a maximum of five dots. This increase lasts
for the remainder of the night. The difficulty of the
roll is equal to double the Trait’s current rating, so a
vampire with Self-Control 4 has to roll an 8 to gain
another temporary point of Self-Control. Only one
Virtue can be increased at a time. (Lore of the Clans, page 201)
Emphases mine. The first bolded section states that the object of Bliss may increase one single Virtue-- this is indicated by the conjunction or. That phrasing means that only one Virtue can be increased with an application of Bliss-- or is exclusive. It would then be redundant to include the second bolded section: the description already limits the increase to a single Virtue out of the three possible scores to raise.
We can assume that it simply is redundant, of course, but I don't see more support for that interpretation than that it isn't redundant. And if it isn't redundant, then we have to parse it to see what it means.
And there I see only one possibility: the word increased is used here as an adjective indicating aspect (in the grammatical sense), and not as a transitive verb in a passive sentence. The latter interpretation is the form that conveys redundant information, and so if we're assuming no redundancy then that construction is excluded already.
Wow, that sounds boring! Why does it matter? As an adjective positioned against the auxiliary verb be, increased indicates the duration and completion of an action without describing its position in time. That reading suggests that only one Virtue can be in a state of having been increased at any given time. (Sidenote: this interpretation does suggest that repeat uses of Bliss can be applied to the same Virtue-- you can keep trying to add temporary dots until you hit 5).
So the operative question is, at this point, "is the second bolded section redundant?". That's going to be a judgement call since, as at the top of this answer, the rules as written are not clear. My personal feeling is that TTRPG rules are generally meant to be clear and concise, certainly within a single paragraph. And so, because there is a plausible reading of the second bolded section which would not be redundant, and would also be consistent with all other rules information and English grammar, we should prefer the interpretation in which that section has meaning to add to the description of Bliss.
This is pretty thin evidence, and so is little better than just picking the interpretation you prefer. But it is more evidence than I can identify for the alternative.
In conclusion, the ambiguity of the written rules here means that there is no obvious way to choose whether at a time means per use of Bliss or per in-game night-- the interpretation does not default to one or the other. For this section, I prefer the per night interpretation only because it has more than zero evidence to support it.
2. How do rules function in VtM overall, and how would stacking Bliss across multiple Virtues fit that?
This is easier than (1). While VtM is far from the most balanced game out there, I can't think of any similar powers which are better-defined RAW and lead to such unbalanced results as stacking Bliss across multiple Virtues would.
There aren't all that many ways to increase Virtue ratings outside of spending precious experience points, and some of VtM's core mechanics revolve around Conscience/Conviction and Self-Control/Instinct. Gaining or losing dots in those is a pretty big deal.
A Storyteller can refuse to include characters that would be necessary for Bliss to be available in a chronicle. But if they don't do so, it seems out of sync with the rest of the game to make it (relatively) trivial to have all of your Virtues maxed out all the time. Particularly if the Storyteller allows it to be as easy as listening to a mix tape for ten minutes every night.
I could go into more detail and suggest some other comparisons, but honestly VtM's overall lack of balance makes me think that that's not worthwhile. There are plenty of ways to optimize characters that can break the game, which is not exactly ironclad evidence that it can't be broken in this particular way as well.
The following assumes a Humanity rating of at least 5 dots throughout: it would cost 10 freebie points to max out Virtues during character creation. That's possible, though it takes 2/3 of the typical number of freebie points available and just under half of the maximum number of possible freebie points. During play, maxing out your Virtues would cost ~28 experience points (a wobbly number to account for Storyteller variation in how and when Virtues can be increased). Bliss costs 15 experience points to learn (assuming you already had the prerequisites). And, if using Bliss on other characters, that 15 point investment can generate the same value over and over again.
It's up to you, as the Storyteller, if the risk of losing Willpower to a botched Bliss roll is enough to rein Bliss in while allowing it to pump all Virtues each night, especially if your NPC using the power is statted to make botching less likely.
In conclusion, broad comparisons with other VtM rules suggest to me that allowing Bliss to affect more than one Virtue per character per night is more than its cost suggests it should be able to do. If you believe that the rules were aiming for rough balance, then consider not allowing Bliss to work that way.
3. How do you want things to work at your table, and how would one interpretation or the other of Bliss support that?
More than any other TTRPG I have any particular knowledge of VtM requires a huge amount of adjustment and polish from the Storyteller to make the mechanics work smoothly. This case is a fine example of that!
Your NPC that knows Bliss is... an NPC. As a result, they probably don't have the same concerns that your players' characters will have, and the story (probably) won't really be about the NPC. That's meaningful because the NPC won't have the same mechanical concerns or needs throughout the game as your players will.
The example that first leapt out at me is: what does the NPC care if they lose Willpower? That's a big deal for PCs, but NPCs are involved in notably fewer rolls than PCs and tend to be purpose-built for those rolls that they do make.
So the PCs are likely to be pretty cavalier about having Bliss used on them since, for them, it doesn't have any drawbacks. Will the NPC operate similarly? What will having Bliss used on them cost the PCs, and how often should they expect the opportunity? What limitations will you, as Storyteller, impose, and what will your PCs' average Virtue stats be each night?
If your effective preference is to limit the Virtue bonuses your players can get, then it might make sense to simply interpret Bliss' ambiguity in the way that is more limiting: you can only buff one Virtue per night, though you can try to max it out. If you don't care about the Virtue stat ratings, but want the NPC to charge a high price for the service, than it might be totally irrelevant if it can buff multiple Virtues per night.
In conclusion, a RAW ruling on this issue in a vacuum isn't so important. You will impose any number of restrictions or liberties around the power anyways, which is 100% within your ambit and responsibility as Storyteller. Effort in determining the most RAW possible conclusion may be misapplied.