Odds are it's recommended to use 7.4s by UK sites but that's more down to warranty than anything else.
11.1s can be used on Krytacs if they have a MOSFET and ETU, just keep the C rating lower (15-25C is normally used) I'm pretty sure the Vector does have both.
Personally I use 11.1V 15-25C on my Krytac MK2 CRB (completely different gun style, I know, but same brand, and contains MOSFET and ETU), but if you really aren't sure/worried about the battery killing your gun, 7.4s will give you no problems.