Study of closing activity of velum with posterior pharyngeal wall is important in characterizing velopharyngeal(VP) behavior. The measurement of VP activities directly from image frame sequences include frames with inter-syllable pauses, oropharyngeal coupling and other VP variables. This yields significant variance in the measurements. As vowels are the oral sounds, VP valve is viable to be closed and locating frames corresponding to vowel onset point (VOP) helps in accurate analysis of VP activity. In this paper, a method is proposed which extracts the VOP from the speech signal and analyzes the frames corresponding to VOP from video files. The VP activities such as percent closure of velopharyngeal gap, velar elevation (palatal lift), knee angle (velar eminence), velar contact (vertical length of VP valve) and velar height are estimated from the frames of image sequences with and without using VOP knowledge. This method is analyzed in MRI and cineradiography of healthy adult subjects, where the measurements obtained with VOP show less variance compared to the entire sequence of image frames. The clinical significance of VOP based analysis in measuring VP activity for CLP is also interpreted. © 2017 IEEE.