It's the damage taken aspect of BFB, not the damage dealt, I'm kinda surprised I had to clarify this because Casters don't have BFB. Melee LB is affected by culling but we're discussing caster LB in any case.
Dunno how vuln/damage up stacking is calculated or how applicable it is, whether it has DR etc though, being honest I've never really paid attention to it, if you get LBed by a melee and you're not a tank, a range on full hp, or a healer that's reasonably topped off you're going to die regardless anyway.
