WCProductVendorsRegistration::registervendor() is protected. It's ran by WCProductVendorsRegistration::vendorregistrationformprocess() for already-registered users. The problem is that I want to be able to run this via my own code, not only via Ajax from /wp-admin/edit-tags.php?taxonomy=wcpvproductvendors&post_type=product

So 2 requested changes:
1) do not exit or wpsendjson() unless DOINGAJAX
2) allow passing a User ID parameter to vendorregistrationformprocess() so I don't have to filter the result of wpgetcurrent_user()

Our workaround is to instead have a pre-existing Vendor that we use as the Template and I just have to hard-code that Taxonomy ID and duplicate off that, then update the new one's data.