If it came to that, I think Democrats should approve any Republican not directly connected to Trump. It's important that Pence and other Republicans feel safe getting things to that point, and that requires an advance commitment from Democrats. I also believe if it gets to that point that Democrats should allow Pence to pardon Trump as Ford did Nixon. What matters is the end effect of Trump not being President, not every little pissant victory before and after.
But he might not fill the vacancy at all. LBJ didn't during his partial term. It's not the end of the world not to.