User wants to create invoices from the vendor bills and mark those bills as invoiced.
When you create vendor bills, Core automatically creates time and expense entries linked to them. If you approve these entries from their respective screens, you can create invoices from them. Check out this article for details: Vendor bill not showing up in invoices. However, there is no option to mark a vendor bill as invoiced. Instead, Core marks the linked time and expenses as Billed when you invoice them.