Picking Haley or even Scott might get him 1 - 2 % more of the vote. Picking Vance doesn't add anything. Ohio is already red. Probably the most obnoxious VP pick possible. Just a hateable dickhead in all dimensions. Adds nothing in terms of politics. Just another wingnut. Adds nothing in terms of demographics. Just another white dude. Has past of vehemently denouncing Trump which will be used against him.
Although, after the shooting, maybe Trump suddenly realized that maybe he is mortal after all, and he may need a clone batshit wingnut and a generally horrible person if it comes time to have an Acting President while he needs some sort of medical care.