Need some info before purchasing dev license

1) Yes Gravity Forms can be used to register users, including custom user meta. You select which role they get. If you want to use something like "Vendor" then you'd have to use a role management plugin like the Members plugin to create a new role called "Vendor" then configure Gravity Forms User Registration Add-On to use that role.

Files are uploaded to your wp-content/uploads folder and the full path is stored as user meta when using the User Registration Add-On if you configure it to do so. Gravity Forms can create custom user meta, but does not display custom user meta on the users profile page. Gravity Forms only handles the registration form. If you want to show custom user profile fields on the users profile page you'd have to write custom code or use another plugin to do that. It's not what Gravity Forms is designed to do which is just forms.

2) No it is not possible to view all entries created by one user. You can browse all entries, but they aren't grouped together by who submitted them. You can just browse and view entries in the Entries area of the admin. They are independent of each other and have no relationship to other entries other than they were submitted by the same form. You can view the uploaded files by viewing the entry details for the form that was submitted.

3) Admins can view form submissions (entries). They can view vendor data and information by viewing the entries for the Registration form you create. I'm not sure what you mean by custom fields, if you mean custom user profile fields... that is separate from the form entry data as that is part of the WordPress user registration system itself. What you view when viewing entries in Gravity Forms is the form data that was submitted when a form is submitted.