Nasalization of vowels is a phenomenon where oral and nasal tracts participate simultaneously for the production of speech. Acoustic coupling of oral and nasal tracts results in a complex production system, which is subjected to a continuous changes owing to glottal activity. Identification of the duration of nasalization in vowels, and the extent of coupling of oral and nasal tracts, is a challenging task. The present research focuses on the changes in instantaneous vocal tract system response to study the effects of co--articulatory load of nasals on vowels. The dominant resonance frequency (DRF) contour derived from the spectra illustrates the contribution of oral and nasal resonances during nasalization. The extent of coupling determines the dominance of these resonances during glottal open and closed phases. A higher extent leads to more decay of energy for the oral resonances, and hence the vowel spectra exhibits a dominant nasal resonance. A comparison of the proposed parameters is made with the previously suggested measures. Several examples of segments of vowels in the context of nasal consonants for English language for both male and female speakers of English are used to illustrate different aspects of the proposed analysis method.