Background: Atrial fibrillation (AF) is a risk factor for cerebrovascular diseases and vascular dementia (VAD). The aim of this study was to evaluate the effect of the adherence to anticoagulant therapy guidelines in patients with dementia and AF on the risk of stroke/TIA or major bleeding (MB). Methods: In a cohort of 1705 hospitalized patients with pre-existent AF, we observed 193 patients with vascular dementia (VAD). Non-demented AF patients were included as controls. For each subject, we calculated CHA2DS2-VASc, CHADS2, and HAS-BLED scores, and collected information regarding anticoagulant therapy, in-hospital therapeutic failure (TF) occurrence, stroke/TIA, and MB. Results: According to CHA2DS2-VASc and CHADS2 scores, 99.5% of VAD patients had the indication to anticoagulant treatment, but only 69.9% were correctly treated. During hospitalization, MB occurred in 4.66% of VAD and 8.9% of non-demented patients (p = 0.048). In-hospital stroke/TIA were observed in 24.3% of VAD and 0.8% of non-demented patients (p = 0.0001). A similar proportion of TF among patients with VAD and with normal cognition (12.9% vs. 11.2%) was observed. Conclusion: In our cohort, we observed that VAD patients with pre-existent AF were undertreated despite a higher risk of stroke/TIA with respect to non-demented patients.