Background/Objectives: Limited data exist on the nutritional status of Vietnamese children aged 5 years and older. This study aimed to (1) determine the nutritional status and (2) assess the associations between malnutrition and socio-demographic factors among children and adolescents aged 5–19 years old in selected provinces in Vietnam. Methods: A cross-sectional study was conducted on 3055 children aged 5–19 years old. Children’s weight and height were collected to calculate BMI for Age Z-score and Height for Age Z-score to determine the prevalence of overweight/obesity, stunting, thinness/underweight, stunted-overweight, stunted-underweight, at least one type of undernutrition, and at least one type of malnutrition. Results: The prevalence of stunting and thinness/underweight was around 10%, overweight and obesity was 14.5%, while the prevalence of malnutrition was 36.5%, with 19.7% of children experiencing at least one form of undernutrition. Significant associations were found between malnutrition and socio-demographic factors such as age, ethnicity, and the number of household possessions. Stunting was more prevalent among older children and those from lower-income households, whereas overweight and obesity were more common in wealthier families. Conclusions: These novel findings highlight the need for targeted interventions addressing both undernutrition and obesity in Vietnam’s diverse demographic groups.