@Nathan Wolhendler, We deactivated all the plugins except WooCommerce, MultiVendorX, MultiVendorX Pro on your end and added a test plan and when a user is registering through that plan the flow is working fine and he is getting added a vendor with the subscription plan assigned.
Now with the plan that was already added on your end, the issue is happening because after clicking on the register button we are getting redirected to a custom page instead of the checkout and hence the order pacing for the membership plan is not done and the user is coming as pending vendor and no membership plan is assigned to him.
So kindly check if you have added any custom code on your end fore the vendor registration flow on your end with the plan that was already added.
Regarding the emails for appointment products, I have installed the email log plugin on your end and when a user is booking an appointment the email is getting triggered accordingly. Kindly check on your end once again carefully.
For the Product add on module, the module is dependent on the wooCommerce Product add on plugin so you need to have the plugin activated and then enable the module accordingly to have the “Add ons” option on your end.
Kindly check all these and let us know if you need any further assistance from our end.
