I assume you mean customer and not vendor.  It depends, your signature does not indicate what version you are on.  If you have IS Tech updates installed, you can split a payment of a single check among multiple customers.  You would start with the custoemr that has only a credit available.  Enter the total ceck amount to that customer and also select the open credit to increase the total payment amount that can be applied to invoices.  Save without applying to any invoices (since there aren't any) and you will be asked if you want to apply the balance to a different customer.  Say Yes and then indicate the next customer and apply payment to invoices and if there is still some left, you can move on to another customer.
 
If you do not have current IS Tech updates installed, you will have to monkey around with vouchers and credits in AR-B to effectively use the credit balance in customer A to pay invoices in customer B