The Doctor and the Vigilante are both pretty important Town roles, however, I do not think our current lack of a Vig is such a great loss. The fact that we lost the Doctor is the real problem...
You should most likely save this role for when you're completely sure as it's a risk; however it's going to be hard for you to survive the night. Hopefully we have another protection role somewhere to give Mafia a dilemma on who to kill.
While some other protective role may indeed exist, I think the probability of that is to small to rely on. Because of this I definitely think that MF should use his ability on someone today. We have nothing to lose by going through with this action since MF is bound to die anyway. Besides, if our Desperado
does hit scum he will survive, and thus he may even survive till endgame since he basically becomes a VT after that (therefore not a threat to the Mafia except for the fact that he is confirmed Town).
As for who would be a good Desperado target, well, I have nobody specific in mind since we are currently so early into the game. The most important thing though, is to use the ability this day. If you do not use your ability in time, MF, I think you have an even greater risk of dying.