http://www.biopac.com/Manuals/dv%20converter%20advc-55%20user%20manual .pdf
Page 13 has the guide for the dip switches.
You can also test if you get a better result by applying set up or otherwise in switch 2.
BTW - how old are the VHS tapes and what do they look like before digitising them?
Are they PALor NTSC originally?
(The box wont do a great job of converting them either so stick to what they are ).