Anyone seen this before? Signed this FA mid season and game wont let me assign him anywhere. Waved him, hoping that would clear it up, but nope

"Fausto Vega does not have a contract for the current season. Once he does, through arbitration or long-term signing, you will be able to assign him.

Maybe try and sign him to the following year and see if that changes anything.. Not a good plan but it is a plan. He has no options maybe he has to sign to ML team? Otherwise no idea.

If Fausto Vega does not have any options, he can only be assigned to the ML roster. Your franchise already has a full 40-man listing so, I guess, somebody will need removing (any level below ML) to free up a slot. The WIS algorithm will then know an assignment can be made whether with a ML demotion or just adding to the Big League team if roster expansion as occurred.