If I am posting a bill, I have classes switched on. the VAT and Creditor side don't get posted to the class. Same if I pay a bill the payment doesn't go to a class.

Hi fiona-drape-offs, thanks for joining the Community - classes let you track income and expense and are set at a transaction or transaction detail-line - they post against the purchase/sales category or item and not the default VAT or credit/debtor accounts. For further info on classes in QBO, please see here or get back to us below. :)