Background/Objectives: Optimal infant and young child feeding (IYCF) practices are crucial to addressing the double burden of malnutrition (DBM), encompassing undernutrition (including micronutrient deficiencies) and overnutrition. This study examined the demographic and socioeconomic determinants of IYCF practices, and their impacts on the DBM among 2039 Vietnamese children aged 6-23 months from the General Nutrition Survey 2020. Methods: Thirteen IYCF indicators recommended by the WHO/UNICEF were evaluated. Associations between IYCF indicators and outcome variables were assessed using logistic regressions. Results: The prevalence of stunting, underweight, and overweight subjects was 10.9%, 5.6%, and 3.1%, respectively. Low serum zinc affected 56.7% of children, while 14.3% had low serum retinol, 31.2% had anemia, and 34.6% had iron deficiency (ID). Only 36.7% of children achieved minimum dietary diversity (MDD), and 29.0% achieved the minimum acceptable diet (MAD). Children from the younger age group (6-11 months), ethnic minorities, those living in rural/mountainous regions, and poorer wealth quintiles had reduced odds of meeting IYCF criteria, including MDD and MAD. Infants meeting MDD had reduced odds of stunting [adjusted odds ratio (95% confidence intervals): 0.61 (0.41, 0.92)], and ID [0.69 (0.54, 0.88)]. Children meeting MAD had reduced odds of anemia [0.72 (0.57, 0.91)], ID [0.66 (0.52, 0.84)], and low serum retinol [0.63 (0.41, 0.99)]. Continued breastfeeding (12-23 months) reduced the odds of being underweight [0.50 (0.27, 0.92)] and of having low serum zinc [0.70 (0.52, 0.96)]. Adequate minimum milk feeding frequency had increased odds of being overweight [3.33 (1.01, 11.09)]. Conclusions: Suboptimal IYCF practices were significant predictors of the DBM among Vietnamese children, with evident age-specific, geographical, and socioeconomic disparities.