Vincent's Limit Break will overwrite any previous effects that set the creature's power and toughness to specific numbers
Eirdu, Carrier of Dawn / Isilu, Carrier of Twilight Eirdu, Carrier of Dawn grants convoke to cards in your hand that were not designed to have cost reduction, and Isilu, Carrier of Twilight gives you boardwide persist
Bounce somewhat scales well with more bounce and so Riftwing's bounce aspect is that much more potent when you have more of a bounce theme going on
I think it could also serve as a 'finisher', where you just needed an extra body to get over the wall
Tormod is definitely the stronger of the two because black works better with the archetype and zombies have considerably more support that spirits, but you'll realistically play either one in your gravebreak deck, assuming your colors support them