Can a repo man come onto private property marked no trespassing to repo a vehicle?

They can and they will. If the car is in a garage, they can't open the garage and take it, but if it is sitting in the driveway they can.

Yes, they can not unlock a lock, go into a closed garage ( well legally) but to be honest even that does not stop a lot of them, and after they get it, it will be their word against yours how they got it.

Can a repo man come onto private property marked no trespassing to repo a vehicle?
Anyone can come onto property marked, "No Trespassing". They must leave when instructed to do so. A more effective sign might be, "This Property Under 24 Hour Surveillance". You need to lock the vehicle in a building the repo man can't find.