The obvious way to do it would be to store the table name in a hidden field:
<input type="hidden" name="table" value="vendor_1">
However, this is generally not recommended, as someone could edit the HTML locally to change the table being submitted to.
Password protecting is probably essential, if you don't want your system to be abused. There are lots of ways to achieve this. One relatively easy way is to set the have a login form to accept a username and password, and to store these in _SESSION variables while the user is logged in.
You should probably set up a table to store the Username, Password and the Vendor table names. Then it will be a relatively easy matter to look up the table name for the logged in user before inserting the record to that table.
Hipe that helps,
James