Venus will have to serve much better than she has in her five matches so far to have any chance against Kerber I think. Kerber doesn't miss much off the ground and is very hard to overpower from the baseline (see Serena in Melbourne) so the match will test Venus' endurance if she can't keep the rallies short. Shvedova and Suarez Navarro were generous with a consistent stream of errors against her, but I doubt Kerber will be today.
Vesnina has a good ground game and serve, good court sense, but if Serena serves well she has no chance.
Edit: Fun quirk of the scheduling: Vesnina (with Makarova) faces Serena (with Venus) in the doubles after the semis are done with, out on Court 2.