Introduction: The preferred PK/PD goal for vancomycin is now AUC_0–24/MIC-guided dosing, which requires two blood samples, additional time, increased cost, and possibly specialized staff, potentially limiting its use. Our study compared vancomycin target attainment using trough versus single-sample Bayesian AUC_0–24/MIC in critically ill patients, using a validated online calculator. Methods: This retrospective cohort study included adults aged ≥18 years with stable renal function who were receiving vancomycin. Exclusions were age under 18, unstable renal function, hemodialysis, missing data, or non-steady-state vancomycin. Steady-state troughs were obtained, and AUC_0–24/MIC was calculated using ClinCalc. The primary endpoint was discordance in target attainment between trough (15–20 mg/L) and AUC_0–24/MIC (400–600 mg·h/L). The secondary endpoint was AKI within 48 h of initiating vancomycin. Results: The mean trough was 14.8 ± 8.0 mg/L, and the mean AUC_0–24/MIC was 499.7 ± 201.3 mg·h/L. Trough levels were within target in 20%, and AUC_0–24/MIC in 67%. The 3 × 3 cross-tabulation showed a significant association (χ² = 27.33, p < 0.001), but only fair agreement (Cohen’s κ = 0.215) and a biased disagreement pattern (p < 0.001). Thirty-eight patients (38%) had “false alarming” results: trough < 15 mg/L despite AUC_0–24/MIC ≥ 400 mg·h/L. Three patients (3.0%) showed “falsely reassuring” results: trough 15–20 mg/L with AUC_0–24/MIC > 600 mg·h/L. The odds of supratherapeutic AUC_0–24/MIC > 600 mg·h/L were tenfold higher in patients with trough 15–20 mg/L than in those below 15 mg/L (OR 10.24, p = 0.048). Conclusions: In critically ill ICU patients, calculator-derived AUC_0–24/MIC showed significant discordance with single-trough vancomycin monitoring, primarily indicating apparent under-exposure by trough criteria in patients with adequate AUC_0–24/MIC levels, which may lead to unnecessary dose escalation.