The risk of the HIV epidemic spreading from high-risk groups to the general population in Vietnam depends on sexual risk and bridging behaviors between high- and low-risk individuals. A cross-sectional study was used to describe sexual activities of youth aged 18–29 years. Nearly half (41.4%) were sexually active. Premarital sex was reported by 43.3% of them; 78.3% of sexually active males and 13.5% of sexually active females. Multiple sex partners were reported by 31.0%; 56.7% of males and 9.2% of females. Almost 27% of males and 5% of females engaged in sexual bridging behaviors. Being unmarried was significantly associated with having sex with non-regular partners. Being unmarried and early age at first intercourse were associated with having sex with a sex worker. Consistent condom use was high with commercial sex workers but low with regular partners. Education to delay early sexual debut, increased employment, and strategies to inform young sexually active people to adopt safer behaviors are urgently needed.