The best way to visit Da Lat is by tourist bus or private car. It takes you 6 hours to get there and the ticket cost 10$ per person (Tourist Bus) and 150$ for private car.
For route to Vung Tau, it's much closer to Ho Chi Minh city than Da lat and the best is by bus (5$).
You can also check the transportation here:
Vietnamtravelrates.com