HI Ari,
To achieve defaulting a field you should write additional code in the modifiedField() method like mentioned by Ievgen. Note that the table VendBankAccount can contain more than one bank account. It is possible to setup a default bank account on the vendor. Probably better to default it from the vendor default bank account.
Also the relation shouldn't contain only AccountId field, but also the vendor account field.
Last comment: Are you aware there is already a field for the vendor bank account on the journal lines?