I've read a few litRPG and webnovels that use this exact setup, and honestly, the rewind function is less about romance and more about survival optimization at first. The protagonist, usually a lone vampire or a newly turned one, gets a do-over from the start of the collapse. The first few cycles are just figuring out where the safe zones are, when the big monster waves hit, which factions turn on each other. The 'harem' forms almost accidentally because you keep saving the same people from dying in specific, scripted disasters. You learn their backstories over multiple loops, which lets you say the perfect thing to gain their trust in a single timeline. It turns emotional connection into a puzzle you can solve with enough resets.
That's also where the darker, more manipulative side can creep in. If you know someone's traumatic secret from a previous loop, you can exploit it to make them dependent on you in this one. The power imbalance between the looper and everyone else is huge, and in a vampire story, that plays right into themes of predation and control. The appeal is watching the protagonist wrestle with that—using a cheat code to build a family, but always wondering if any of the bonds are real or just a product of perfect information.