Background and Objectives: Vestibular schwannomas (VS) are slow-growing tumors of the vestibular nerve. Gross total resection (GTR) is recommended for large tumors (> 4cm), brainstem compression, cranial neuropathy, and hydrocephalus. In select complex cases at our high-volume skull base center, subtotal resection (STR) is pursued for various clinical factors. These patients may require adjuvant radiotherapy (ART) if residual tumor progresses. We sought to evaluate tumor control following STR of VS and identify risk factors of future ART need. Methods: A single-center retrospective review (2009–2019) identified patients with VS with complete data who underwent STR without prior treatment. Tumor volumes were extracted from contrast-enhanced T1-weighted volumetric MRI by semi-automated segmentation using 3D Slicer. Clinical and procedural data, including House-Brackmann scores at discharge and 90-day follow-up, were reviewed. Postoperative tumor volumes and progression-free survival (PFS) were calculated. Multivariate logistic regression assessed risk factors for ART. Results: Among 1119 VS patients, 38 (3.4%) patients underwent STR. Of the STR patients, 15 (39.5%) required ART. Preoperative volumes did not differ significantly. In STR-alone patients, the average residual volume was 15.26±3.54%, while in STR+ART patients the average residual volume was 37.53±12.33% (p<0.001). The smallest residual requiring ART was 23.4%, while the largest residual not requiring ART was 20.6%. Median PFS among ART patients was 29.67 months. On multivariate analysis, percent residual volume independently predicted ART need (AOR 1.28, CI 1.06-2.29, p=0.002). Conclusion: STR is viable in select complex VS cases. Residual VS volume ≥20.6% may confer increased risk of progression and necessitate ART. Close postoperative surveillance over 2-3 years is warranted.