I think Bernie needs to go more centrist with his VP pick to try and bring that side of the party with him. Picking someone like Klobuchar might ease some anxiety and help out in the rust belt.
Then he turns off part of his base, because it means "if I leave office the next person won't advance my policy priorities".
As for the Rust Belt, the biggest issue there is trade (and specifically being more protectionist), and Sanders already has that position.