I'm trying to make a VATS based character at the moment, and even though i have 120 AP, I can still only get 6 shots with a Laser Pistol.
Logic would dictate that

15 x 6 = 90 AP

Therefore I should be able to get off two more shots with the pistol, right? But nope.

Is there something I'm missing here?

Do you meet all the requirements? energy and STR?

Laser Pistol STR and Skill requirement are 1 and 0

So it can't be an issue related to that.

Just made a new lvl one character with 92/92 AP and used a Laser pistol, and only could get 5 shots in VATS. 5 x 15 = 75 AP

Has anyone else had this problem? or is it just me?

Do you have trigger discipline trait? That thing increases ap costs by 20% and would explain why you get so few shots in vats.

15 x 1.2 = 18

120 / 18 = 6.66... meaning 6 shots in vats like you are getting in your first example

and

92 / 18 = 5.11.... would equal 5 shots in vats like in your second example

Just went into console to test some things, i modav changed AP value and found that the Laser Pistol fired 6 shots only at 108 AP
I then changed Agility with modav and it had no effect on the AP score.

Ah thanks, thats the reason thanks for the help! I had no idea that perk increased VATS costs.

Does the Fast shot Trait also affect VATS?

Yeah, the full effects of the traits are:

Trigger Discipline

20% slower attack rate
20% increase in ap costs
20% decrease in weapon spread
20% increase in vats accuracy

Fast Shot

20% faster attack rate
20% decrease in ap costs
20% increase in weapon spread
20% decrease in vats accuracy

The vats accuracy bonuses and penalties are the same type as gunslinger/commando/sniper bonuses meaning that base 40% chance to hit becomes 48% whith TD and 32% with FS for example.