Not a guaranty this will work with anyone else, I’m just posting what happened to me.
I left Vassili for last and when I went to acquire it it told me I could not buy him because I already had him. Didn’t lose any credits.
Tried to do it again a few more times with the same result.
For some reason I decided to try and replace my entire squad with completely different mercs.
Tried aquiring Vassili again and success! …odd.