Background: Data on the nutritional adequacy of unsupplemented vegetarians in Northeast China are limited. Methods: We compared dietary intake, body composition, and serum biomarkers among vegans, lacto-ovo-vegetarians, and omnivores. This cross-sectional study included 356 adults (all diet ≥ 2 years, no supplements). Dietary intake was assessed using a validated semi-quantitative FFQ, body composition by BIA, and serum biomarkers. Kruskal–Wallis tests with Bonferroni correction were used. Results: Vegans had lower BMI (22.0 vs. 24.6 kg/m²), body fat (24.5% vs. 28.0%), and visceral fat (0.65 vs. 1.05 L) than omnivores (all p < 0.002). Vegans consumed more fiber (38.5 vs. 18.0 g/d) and vitamin C (180 vs. 85 mg/d), but less vitamin B₁₂ (0.3 vs. 4.2 μg/d), vitamin D (0.5 vs. 3.2 μg/d), calcium (520 vs. 720 mg/d), iodine (65 vs. 130 μg/d), and selenium (45 vs. 85 μg/d). Serum vitamin B₁₂, 25-(OH)D, ferritin, and selenium were significantly lower in vegans, while homocysteine was higher. The proportion of vegans with dietary intake below the recommendation reached 100% for vitamin B₁₂ and 97% for vitamin D, whereas omnivores showed excessive sodium (75%) and SFA (70%) intake. Conclusions: In this Northeast China cohort, unsupplemented vegetarian diets offered favorable macronutrient profiles and body composition but were associated with a high prevalence of dietary intakes below recommendations for vitamin B₁₂, vitamin D, iodine, selenium, zinc, and calcium. These findings underscore the need for targeted supplementation and food fortification strategies for individuals adhering to plant-based diets without supplement use in this region.