In VTM a player is given 15 freebie points and the disciplines only cost 7, this seems to mean that you can raise a discipline to 5 dots from character creation which seems grossly overpowered, is there a rule I missed somewhere or is this just something that exists?
Each character starts with 3 dots worth of Disciplines, so if you put all three into the same dot, and then spend 14 of your 15 Freebie points on the same Discipline, you could very well do so. There is a trade-off to be made though.
During the game, buying an additional dot in a Discipline you possess requires Experience equal to the current rank of that Discipline × 5 (or 7 for non-clan disciplines). However, the first dot in any given Discipline costs 10 experience points, which will require you to save up for a longer period of time before being able to obtain a new Discipline in game than it would take to advance a pre-existing one.
There is also this helpful sentence under the Freebie point section that reads (emphasis mine):
These points may be spent however the player chooses — thus the term “freebie” — though the Storyteller is the final arbiter of what she chooses to allow in the chronicle.
In the sytem, nothing forbids you from doing so and then handing the sheet to the GM for approval. Nothing stops you either from taking the maximum ammount of flaws to fund yet another dot in some other disciplin or any other things you feel like you need (like the skill that belongs to the disciplin's abilities on 5 for another 4 freebies.)
However, the GM is still allowed to deny the character or request a background justifying said 5-dot disciplin (and skill). He has the right because he is the storyteller and may just rule such a thing not appropriate (V20 Core p 83):
After the players are familiar with the character sheet, give them a few guidelines about what types of characters will be appropriate for the chronicle. [...] Sometimes a player will attempt to portray a character wholly unacceptable to your plotlines, and you should feel free to disallow this in favor of a character who won’t disrupt the game.
I will have to repeat some other answers, but I am sure that I have something really new to add.
TL;DR: you can do it, but it is very, very impractial for various gameplay reasons and not logical plotwise.
Yes, it is technically possible to raise your single discipline to level 5 at character creation.
Most, or at least a lot of really strong discipline powers require you to roll some Attrubute and some Ability to work. And if you put 14 freebie points in discipline dots, you won't be able to make the respective traits high enough for your discipline to work reliably.
Most level 5 powers are nice supplements for an already powerful character. But they are also very weak if you character is generally very weak. Let's say, you bought level 5 Valeren, Vengeance of Samiel: it makes all of your dice to hit an enemy automatically succeed. However, it is only powerful if you have a lot of hit dice, which normally comes with a powerful vampire. TBH, I doubt any useful usage of Valeren by neonates. Mental disciplines, like Presence, require low generations, which also suck your freebie points.
In my opinion, most level 5 powers are actually just useless. Compare them to low-level ones, and you will see the diffrence. For example, level 4 Protean is a damn good thing, allowing you to be a king of neonate combat. Protean level 5 is not as powerful at all.
If you are looking for a powerful build, you will spend your freebie points not on disciplines. For example, if you have Thaumaturgy at level 5, you will not be very powerful and will lose your sanity very quickly, because you are going to botch Willpower rolls a lot. But if you raise it to just level 3 and also raise your Willpower to like 8 or 9, you will succeed most of the time, and as you use low-level rituals/path powers, it will be very realiable.
As mentioned by other users already, the Storyteller can ban you from going for level 5 powers. I, however, would not, for reasons mentioned above: this decision is stupid, the character will be very weak).
If you play with a Storyteller that gives out a lot of XP, or if you make a neonate that will be then used for generating and ancilla/elder (and thus want to buy as many points worth of XP as possible), I would advise to buy a lot of disciplines of level 1. They all cost 10 XP, so it will be cheaper to raise them.
A good addition would be an advice to look at some character sheets. Notice how many different low-level disciplines do vampires generally tend to have. It isn't good to be a one-trick pony.
I found a way to have a lot of disciplines, and it is totally legit in V20 (page 500 of the Corebook). Say that your character was a ghoul, and then he was embraced after some time. When you become a ghoul, you get 6 more freebie points to spend on your attributes (+1, 1 point left), or abilities (+3), or Willpower (+6), or Background (+6). You also get a free dot of Potence and one dot in some of your domitor's clan disciplines. Then, when you become a vampire, you get a free dot for each Attribute category, +2/2/1 in Talents, Skills and Knowledges respectively, and 3 (or 4 if you are a monstrous Sabbat spawn) disciplines. Of course, if I would be your Storyteller, I would think if I allow such abuse of rules, or is it an intended feature. As for now, I vote for the second answer. Anyway, that would require you to buy the "Former ghoul" merit.
Good luck in your min-maxing! Hope that I helped you.
With best wishes, a fellow min-maxer.