Youths and adolescents are vulnerable to HIV/STIs from unprotected sex. Promotion of young population’s awareness about risky sexual behaviors is essential to develop contextualized interventions. A cross-sectional study was conducted in five Vietnamese provinces to document current attitudes and practices regarding sexual behaviors among youths. The information on sociodemographic characteristics, substance use, and sexual behaviors was collected via self-reported questionnaires. The factors associated with risky sexual behaviors were identified by the multivariate logistic regression. Among the 1200 participants, 73.5% reported having sex in their lifetime, and 48.1% used condoms at their latest sexual intercourse. Participants in urban areas were more likely not to intend to use condoms and had a higher unintended pregnancy rate than in rural areas. Older age was positively associated with not wanting to use and not using condoms. Substance-using participants were more likely to not use condoms. The participants taking alcohol or other stimulants before sex had a higher likelihood of unintended pregnancy. Respondents’ attitudes and practices regarding sexual behaviors were associated with gender and employment. This study indicated that young population’s awareness in Vietnam is high, however, risky sexual behaviors also remain common. Sex-related educational programs about the consequences of substance use, multiple sex partners, and unprotected sex should be developed.