It depends which country you are in but, for example, go to:
http://www.apple.com/contact/
and in the section 'How to buy for Education' half way down the RHS it says:
If you are buying on behalf of an educational institution, visit the Apple Store for Education Institutions or call 1-800-800-2775, 7 days a week from 5:30 a.m. to 4:30 p.m. Pacific time.
Find a telephone and then...
I suspect that you will have to set up a new Apple ID and use it wholly and exclusively for VPP purchases. If you really have to use a PO then there is (or at least used to be) a work round that involved buying iTunes gift cards from Apple with the PO and then redeeming them in the App Store. Do that a couple of times and your finance officer will probably authorise use of an institutional credit card 😁
C.