The Vendor Balance field in the Purchase Orders, Purchases, and Vendor Credit Memos windows displays the total of all unpaid invoices currently in the vendor ledger as of the system date. Even if you are in a previous accounting period and your system date is today's date, the current balance reflects all unpaid invoices as of today's date.
If you select the arrow button to the right of the vendor balance, the current Vendor Ledgers report for this vendor will appear.