It only eats batteries while vibrating, though. Two hours is more than enough. Also, it's still STR/2+2 while inactive, so it's not like it's useless if the battery somehow runs out (which, really, should never happen. Two hours active is a huge amount of time). Normally you can just turn it on then whack people, but if have someone cold, hitting them with a "mere" 6P attack when they can't defend themselves will do the job perfectly well.
If you're concerned about concealability/legatlity, might I suggest putting it in a Gentleman's Cane (Attitude)?
Granted, VB swords are not maximally subtle. Katanas aren't exactly subtle either. If you do want to go for subtle, I would suggest going all the way subtle and using Ceramic Knives.