Hi,
Thanks for the report.
When you're using the HikaMarket registration page, the Joomla account is created before the vendor one.
But, the synchronization is made when a user is created or modified ; so the moment when the user is created, before you're in the registration process, the vendor account is not created (and do not exist, yet) so the synchronization cannot be done.
That's also why, when you edit the user afterwards, the sync is made and the vendor account receive the right user groups.
I will add a patch in HikaMarket registration process in order to read the Joomla account groups during the vendor account registration.
Regards,