Hi A.j.
Excellent advice from registered. I’ll add my 2c as someone who records a lot of free-flowing piano/vocal and then overdubs to that.
It’s definitely possible to use Cubase’s tools to help … but I am strongly inclined to believe that the better option would almost always be to have the drummer practice until they can play along well to the free-flowing piano/vocal. I’d think any drummer that plays live, i.e., plays well without a click track, would get this down without too much pain and misery.
This recommendation/bias is based on my experience that often in free-flowing piano/vocals, the non-quantized aspect that makes it so interesting and personal is not just that the tempo is variable, but that - even within that variable tempo - many of the piano/vocal “hits” are off the grid (intentionally leading or lagging behind the would be tempo track). So setting up a tempo track so the clicks line up exactly with the piano/vox hits (using tap tempo, etc.) is easy enough, but can suck the life out the piece.
The alternative (deciding for each “hit” of the piano/vox whether it is truly played before/on/or after the theoretical tempo click) is often enough so hard that I think it sounds better to humanize the accompanying tracks (by having the accompanyist, drums in this case, practice enough to sound good playing along live) rather than force them to fit a tempo track of dubious relationship to the actual song.
But, if you can’t retrack the drums … creating hit points on the drum track (I’ve read that Cubase 7.x is much better than 6.5 to do this, but I can’t confirm), then using the best piano/vox click track you can make to guide where to slide the drums is something to try as well.
Good luck, hope you turn out to be the hero!
(PS - there are some links in my sig relating to this topic, maybe they’ll be helpful a bit).