Vendor default account is a cost of sales account.How to I set this up?

I can share some information about setting up default accounts, @lessenberry-anim.

The feature to set a default account in QuickBooks Online (QBO) for your vendors is currently unavailable.

I'd also suggest consulting an accountant on what accounts will be affected when processing vendor transactions in QuickBooks Online (QBO). 

You can check these articles for more information:

• Enter and pay bills
• Enter and write checks in QuickBooks Online
• Enter, edit, or delete expenses in QuickBooks Online

I can see how having different options and default accounts for vendors would be helpful in QBO. I'll take note of this. For now, you can visit the QuickBooks Blog to check for new updates and features that we have. 

I'm always here to provide help and answer to any questions that you may have with your account. Have a great rest of the day!