You know they can go the other way? Enter the vendor in SO lines then use Auto Generate in PO module? That may be why it doesn't validate PO# in SO line entry.
From SO Help
Enter the appropriate information in the secondary grid.
If the item is being drop shipped, select the Drop Ship check box.
At the Vendor Number field, enter a vendor number if a purchase order is to be generated for inventory, miscellaneous, and special items. for inventory items, the Vendor Number field will default from the Primary Vendor field in Inventory Maintenance.
Note The purchase order will not be generated if the Vendor Number field is blank.