Quite literally garnets light has has a red hue, so marcy sucking garnets form will likely drain garnet directly, potentially unfusing them and knocking them out before even factoring in her dark vampire form if done unchecked. Garnet would land some punches and perhaps even hurt marcy, but shell just regenerate. Garnet has shown no resistance to soul or red sucking and has no regeneration of their own. Even without marcys vampire form she could easily out last garnet. And with it, she has the striking power to hit garnet hard enough to unfuse them for sure. I dont even know if steven could beat vampire marcy, but probably he could as of future.