If you turn this feature on, it will not just be for Vendors only. It will change the whole QBO file and also you can't turn this feature OFF once it's ON. Also, you will need to create a new Vendor (or Customer) to select the currency you want.
"You can assign a currency to most types of accounts, including customers, vendors, bank and credit card accounts, accounts receivable, and accounts payable."
So, if you need this just on the Vendor side, it may not worth the global change in QBO.