Is it possible for vendors to fill our their own information on Netsuite (W9 info, etc). If not, does Netsuite accept any type of excel/CSV file with this information?

netsuite has a concept of a "vendor center" i believe where you can pay for vendors to have licenses to your NS and access a limited number of objects, but i'm not sure if editing their own vendor record is one of those (nor would you necessarily want them to have direct access). but yes, you can CSV import to many records in netsuite, including vendors.

You could allow them to submit a form to a custom table. You could email each vendor a unique link and they would update the info. You can then update form the table to the vendor’s records