Using density functional theory calculations within the generalized gradient approximation we have examined structure and dynamics of neutral Si vacancies at Si/SiO2 interface. We show that Si/SiO2 interface may serve as a limited sink for Si vacancies. Single vacancy and vacancy cluster defects are substantially more stable at c-Si/a-SiO2 interface compared to the bulk c-Si layers away from interface, mainly due to termination of dangling bonds with bridging O atoms and reduction of interface strain