What can we Grievous Wounds and what can we not?

The way Grievous Wounds works was always too vague for me to fully understand how it really works, if it is working or if its not working. They should make it so that it shows when it's actually affecting enemy players that have Grievous Wounds or at least make a list of things that are not going to be affected by it, I'm still not entirely sure if it actually does anything to Soraka heals, Vlads spellvamp and more healing abilities since I cant tell if its working or not.