@MMK48 

"Would they allow me to add another line in the same Journal Entry and somehow get it into the vendors account?"

Nope.  You can make two journal entries:

#1 - Debit A/P (list vendor), credit Clearing Account

#2 - Debit Clearing Account, credit A/R (list customer)

But, then you need to apply those credits to the invoice and bill.  The instructions in my first do the same thing but in fewer steps.