Why would a solo scum want to claim vig? He has to die soon after.
He was not solo at the time. He had teammates, or did we kill them so fast you forgot? :P

The answer to your question is, the same reason they claim cop sometimes. To survive a lynch (even if only one lynch) and then use the doctor as an excuse as to why they still live.

But this game don't have a cop. So he wasn't gonna crumb cop, right. So he crumbed vig instead, anticipating the game had one.

They have only very limited use out of claiming doctor. Doctors always have to die. If he can claim vig at the critical moment, he can draw a lynch away from someone and onto someone else. Particularly, the town vigilante, if they suspect the game has one.

Why not, if he's town? I'm certain townies have speculated on there being 3 kills before.
With one death? I don't think so.

Why would "lock town" Slaan trade himself to keep Zack and/or GH alive for another day?
They were not lock scum at the time. He was still arguing until Zack flipped that Zack was town, for example. Many people still had skeptical face.

All of those plans got shot to hell when Zack did.