You wouldn’t enter VAT on a purchase order, as it would be up to your supplier to determine the VAT (whether it would be chargeable or not, and at what rate).
You can always amend the purchase order before converting it to a purchase anyway, where is where the VAT rate can be entered.
See this post here:
http://community.quickfile.co.uk/t/vat-drop-boxes-on-purchase-orders/640/2