Greetings, mmankin. 

Thanks for visiting the Community today. I can share some insights on how to add your vendor accounts. 

At the moment, adding vendors multiple address is currently unavailable in QuickBooks Online. 

As a workaround, you can save the vendor's second mailing address in the Notes box of their Vendor Information pop-up box. This way, you can copy and paste the vendor's second mailing address from the Notes box to your bill payment's Mailing Address field.

This way, it would be easier for you to change the address every time you create a purchase order and payment.

While we continue to make your experience with us even better, I recommend visiting our  blog site 

 so you'll be updated on the latest features and improvements in QBO.

For future reference, I’m attaching some helpful article about 1099 that you can check on:  Prepare and file 1099s using QuickBooks Online. 

Please get back to the Community if you have anymore questions about QuickBooks. I'm here to help you whenever you need further assistance. Have a great day!