There is a Company Preference option under the Checking catagory that allows you to autofill your account number for the Vendor being paid into the check memo. If all you want in the check memo is the account number, make sure to checkmark this selection and record your account number for the Vendor in the Vendor's defaults under the Additional Information tab.

If you want the Memo line from a Bill to autofill in the Memo line of the payment check, you need to make sure this option is unchecked. Make sure to record everything in the Bill memo that you want transferred to the check, including your account number.  With this preference unchecked, the account number will not autofill, even if you have it recorded in the Vendor's defaults.