Virus are some of the most powerful civys, if they have common sence. Last mafia, viruses went ballistic and aimed for random guys(BC was plain bad, and terro realized only a little too late of his mistake). However, in other mafias, viruses have been very effective(2 Fgs killed in mafia 15, for an example). Viruses are supposed to think in depth their movements and make a decition carefully, not go ballistic or rush up the kill and end the investigation after the kill.
I think vigilante is a little lame, all it gives is an extra vote(not so useful as other vote manipulation skills).
One other option is to change the virus ability to killing the target at the end of the next round. The virus would die the round they used the ability, people would know that someone is going to die the next round, but only the effected player would know they had been targeted. There would be no saving them. They could just reveal that they were the targets but they may not want to for whatever reason, and others may not want to believe them for whatever reason.
I dont really see how does it change something for the virused guy, it seems like an unneeded change.
You choose dema, what do you want to do with viruses?