To do so you'll first need to enter the vendor part number for items into both the Reorder Number field under the Supplier tab and enter it again under Aliases.
If you have AutoGen installed, you can enable the option to copy Supplier Reorder Number to Alias from Item Properties and Purchase Orders. See AutoGen Help for details.
If you have DRS Power Ops installed (Bicycle Bundle, RMS Toolkit, Matrix Master), you can easily build PO's with multiple search and tag by vendor part numbers, and more.
Update: RMS 2.0.0126 and higher locates vendor number (supplier reorder number) when the supplier is assigned to the PO. However, you will still need to copy SRN to Alias if you want to enter vendor part numbers at POS when they are not assigned as Item Lookup Codes.