I am currently working on a profile for my website. I am trying to make it so that I can save profile data that is stretched across multiple tables.

For example I have Personal info which is the main User Table. It holds personal information about the user.
Then I have separate tables for the usernames they have, as I will have other information pertaining to the user under that username.

When trying to save the information to my database. I get this error: A referential integrity constraint violation. I have tried making individual forms for this, but I don’t see that working. My question is, do I make a HTTPPOST method for each table I want to save data to or can I do it in just 1 POST method?
You can make an POST for each form you are posting to the server. If the formfields in that form match a domain object (at least the primary key), then you might be able to use your database objects in this way. It depends on how your database objects are constructed.
Yes, create a view model that contains members for all of the formfields on the form. Be sure to include hidden formfields for whatever primary keys you need for database lookups. When you POST, map the view model members to your database object.