Vung Tau is hot all year — daytime temperatures hover around 30 °C with high humidity. The dry season runs from December to April and is the most reliable time to visit: blue skies, calm seas, comfortable evenings. The wet season from May to November brings short, often violent afternoon storms but otherwise plenty of sunshine; the storms tend to clear by evening.
The biggest issue is not weather but crowds. Weekends are packed with day-trippers from Ho Chi Minh City — book a hotel ahead and expect a queue for seafood. Tuesday to Thursday is consistently the quietest window. Public holidays, especially Tết and the April 30 / May 1 long weekend, are best avoided unless you specifically want the scrum.