Membership 2 Custom Request and Default Membership not working

I really need some additional information from a user to filter what memberships he can subscribe to but I don't want the whole BuddyPress codebase.
Most simple example: A users can only subscribe to a 'pro membership' when I have a VAT number of the company of the user. I have the API Documentation of the Membership 2 plugin but not sure how to take this on. Should I be extending user profiles or use extra fields for memberships, etc?
At this stage, I've been trying the registration plugin from Gravityforms as well. It would be nice if you could point me in the right direction for achieving this.