PC on this bow with strange inherent mod

hey everyone. I found this bow but it had a weird inherent mod. a +15% with a vamp degen
I've been playing gw for about 3.5 years now and i havent seen this mod before.

Its a r13 Flatbow
Mods:
+15%/-1 health regen

string adn grip are
vamp +5/-1 and +7 physical damage

Don't drop too often but they are not all that strange. Don't sell for much, not too much use for it...