If I use Victini V's "V Bullet" attack or Victini VMax's "Max Victory" attack and my opponent’s Active Pokemon is a VMax Pokemon, does the attack do more damage?
Yes, per the Rule Book, Pokemon VMAX are always considered to be Pokemon V.
Source: Battle Styles FAQ; TPCi Rules Team (2021-03-18)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH5_21
SWSH5_21
SWSH5_22
SWSH5_22