Unlocking variants

I know it's probably been discussed MANY times, but I want to avoid spoilers and try to unlock some of villain (and hero) variants on my own.

I did look at one or two variant unlock conditions (for Omnitron and one other villain), and they both said something like ('defeat the base variant and then do XYZ in another fight'). For example, for Baron Blade, which is my only unlocked non-vanilla villain, I had to defeat 3 specific Citizens in one round.

Now, does this 'other fight' have to happen immediately after I defeat the base version of the Villain?

No, for any unlock that requires multiple games, you can play games in between.

Just make sure you remember you did the prereq game so you aren’t repeating yourself needlessly.