There are several bus companies that run to Vung Tau from HCMC daily and usually takes about 2 or 3 hours. Your hotel can handle the reservation or head to Pham Ngu Lao where there are a bunch of agents that will book a ticket for you. If not, then head to the Mien Dong Bus station where high speed coach lines leave every 15 minutes. The same works with buses heading back to HCMC.
Some may want to take the slower land-route to and from Vung Tau. The main bus station located just off of the city centre houses a dozen stands ranging from taxi vans to large coach busses, sending you Saigon-bound. The ride is a short two hours and should cost you VND100,000-150,000. A bus departs every half-hour.