Had a longer look at them and here's my thinking on each
Vrignaud - I think has the potential to play any role in the DEF/CDM positions. Any crossing in the double figures makes me excited, and his mentals and physicals are solid allround, so any CB, FB/WB and CDM role could do based on your tactics. I also thinkhe has the potential to be a handy player if you wanted to change from a traditional WB to a IWB, assuming his ball playing related attributes improve over time.
Miath - I agree Mezzala is his best current role, but I agree with you that RPM should be his endgame. His determination and aggression does also wonder whether he could also be handy in the PF-S role if you wanted to be a bit leftfield, however in some playmaker role is my overall suggestion.
Amara - LW/RW/ST I think is the best for him. Got some work to do to be effective, but I'd focus on those areas.
Bensmann - Agree, even with his low determination and casual mentality, he does excite me. FB/WB/LM/RM and the CDM role to use that potential engine would be my thoughts on that. I did do a Vaduz save a few FMs ago so any Liechtenstein player gets a bump in my eyes!
Mohammed - basically the same as Vrignaud but a little less exciting.