How can I prepare slightly uneven tiled floor for vinyl 'planks'?
I have used this product myself:
DAP floor leveling compound



It is thick enough that it does not actually seek self-level, but it can be used with a good straight scrim to get to "TRUE" which means "flat not necessarily level/plum".

This should be perfect for you.  Follow up with a 1/8" layer of plywood if you like.