My character isnt a vampire, but I was grouped up with one tonight, and noticed an interesting new bug. The vampire used feed on an NPC (at least I'm pretty sure it was either the feed or their first drain/leech ability, dont know much about the vampire lines), and while the NPC was stunned for the 3~ second feeding duration, none of my attacks dealt any damage to it. Used wrecking blow, surprise attack, and impale all during that 3 secondish animation/stun and the target took no damage from any of them.
PS4 / NA
M'asad - Khajiit Nightblade - Healer
Pakhet - Khajiit Dragonknight - Tank
Raksha - Khajiit Sorcerer - Stamina DPS
Bastet - Khajiit Templar - Healer
Leonin - Khajiit Warden - Tank