If i say [va] will the vocal folds be continuously vibrating through the [v] or will they start vibrating at the initial onset of [v], lower in amplitude, and then start again at [a]?
I assume you are asking about the magnitude of the source waveform, not the mouth output (acoustics). You'd need to hook yourself up to the machinery to find out for sure. The chances are reasonable that there is an initial voiceless period during the fricative. I don't know of any systematic studies of amplitude-dip within the fricative. There is a bit of a problem in defining "at [a]", since the vocal tract opens continuously. Before the mouth is fully open, it is half-open, and a quarter-open (etc.) all of which affects the acoustic waveform and the oral pressure (which affects the ability of the vocal folds to vibrate).
The paper "Acoustic and perceptual characteristics of voicing in fricatives and fricative clusters" (JASA 91, 2979 by Stevens et al.) does attempt to estimate the amplitude of voicing from the amplitude of H1, but does not include initial context. In intervocalic contexts, there are multiple rates of decrease and increase of amplitude. The little vertical ticks are the points where amplitude has decreased by 10 db, which they use to define the interval "duration of glottal vibration within the fricative".