The audio would most likely have to be a composite of the LD and the VHS - and perhaps even the DVD as well - taking the best of each. The VHS audio is hardly going to be relatively optimal quality compared to the LD (in theory anyway), so it would be good to use the latter whenever possible.

Probably the easiest way to do it would be to put the DVD's video (either before or after you've converted it in Procoder) into an editing software timeline (e.g. Premiere, FCP if you have access to these), together with the audio for each of the three versions and then export the audio once you've picked all the different bits and put it together.

If you don't feel confident enough doing this, but were willing to post me the source files on data DVDs, I'd be more than happy to :)