1) When using Synth 1 via Vestige, press the wrench symbol.
2) A basic gui for Synth 1 will appear, with lots of old school looking dials.
3) Find the modwheel dial. (hopefully, it is there and you find it.)
4) Ctrl+click hold the dial (modwheel dial), you want to automate and drag into the automation blocks
in the song editor etc.
5) Make your automation pattern etc.
I hope my instructions were clear. Tell us if you get through.
UPDATE: I found this post. Check it out, it might help you.
viewtopic.php?t=1719