Custom field on registration form won’t working.

@satyajitghosh, if you have added the form that should display in the frontend.
Now as you are facing this issue, please do contact the team over forum, so they can help you out – https://wc-marketplace.com/support-forum

Also, to display a message after successful registration, you have to do custom code. May I know where do you want to show this message i.e. in which page.
Just to keep you posted, if a vendor is successfully registered, we also send them a mail staging the same.