Hi All, While uploading historical vendors using CSV, can we map contacts, email, and phone number of company vendors to line items in the Relationships.

Yes, but not on the same import. You have to upload vendors first and then contacts separately. Check out https://suiteanswers.custhelp.com/app/answers/detail/a_id/66604/loc/en_US

I have imported vendors. Do I import all of them later and select Add or update?