There are two places that get checked to determine if a posting journal file has been cleared. One is a 'flag' in the data, and I suspect that it has been set for you; you can see this if you attempt to print a posting journal and get a message that 'there is no posting journal to print'.
The other place is the size of the posting journal data file itself. I have seen occasionally that someone has printed and purged a posting journal, yet the data file itself was not cleared to zero bytes for some unknown reason.
Use Windows Explorer and browse to your data directory. Look for files that start with the letters APPOST. You likely will have a lot of them; for invoices, manual checks, system checks, adjustments, check reconciliation and error batches for all. If any of these files have a size larger than zero, yet you cannot print the posting journal to clear, then this is the culprit for your situation.
As long as you are confident that the reports have been printed, you may delete or rename these APPOST* files. They will be recreated the next time you post a batch. This should also clear up your situation with the changing fo the account set on a vendor.