Vessery’s ability states: “...after you roll attack dice, you may aquire a lock on the defender.”
So V engages a target that is locked by a buddy. Rolls attack dice (3 blanks). You decide a lock would be handy right now so you acquire one and spend to reroll the dice.
The target still has the lock from your V’s buddy, so do you now get to aquire a lock.
Basically, it comes down to a core mechanic. Either a “reroll” is considered a subset of “roll” in the sense that and any rules about when a play may “roll” dice apply to rerolling dice. Or “reroll” is considered distinct and anything rule that applies to both a roll and a reroll would have to be stated specifically.
I have not been able to determine which is the case.