Just tested it.
Played with Sion, took turret hits to 10% health, and popped ult to attack blue golem. Regenerated 54-55 HP per hit normally, had friend hit me first with Ignite and Executioner's Calling after that wore off. Kept regenerating for 54-55HP per hit under both circumstances.
So I guess that's a no, Grievous Wound doesn't affect lifesteal.
Didn't test spell vamp but I'd imagine that's the same deal.
Originally Posted by Kyrokiza
yes, I've used it against nasus, ww, and alike
They are special cases; their lifesteal is an innate ability. Much like Swain and Fiddlesticks' spell vamp, this (I guess) isn't considered "real" lifesteal/spell vamp and is therefore affected by the Grievous Wound debuff.