Vervain is the drug/herb that weakens vampires in the TV series The Vampire Diaries which I have watched since I was a kid so my huge association with that kind of ruins it as a name for me.
If I didn't have that association though I'm still not sure I think the sound of it. As previous posters have mentioned, the appearance of "vain" in there just isn't a virtue I'd want to bestow upon a child.
http://www.behindthename.com/pnl/117870?sort=rank&pop=ew^Top 2 = Alice, James / Henry ~~ Bottom 2 = Courage and Indiana (m)