The vendor class records assigns default cash and payable accounts to a new voucher based on the vendor class of the vendor.  Most foundations have only one class- admin - and the default checking and accounts payable accounts are filled in on that record ( found in File Maintenance > Accounts Payable > AP Code Maintenance\Vendor Class ). 

If you have filled out the vendor class record, when you process admin vouchers, the line item accounts for cash and payables will fill up automatically.  If you have filled out the expense account on the vendor tab for the payee record as well, all three values will fill in automatically.

You can have different vendor classes for different funds that might pay administrative expenses.  However, if a particular vendor is primarily paid out of the operating fund, the vendor class should be the operating fund one.  If a particular vendor is primarily used for events from a particular fund ( like a caterer), then please do add another vendor class for that fund and select the expense account from that fund as well. We advise you to always fill out an expense account on admin vendors to make data entry more efficient.

Most foundations have only the admn and grnt vendor classes.  The grnt one is there to ease in reporting. Grant accounts come from the fund class record, not the vendor class record.