A lot of the time, paypal automated system will not allow new accounts to add money without having valid ssn info. It's a bitch and supposed to help prevent people from buying illegal services anonymously. Stupid, I know. One way around this is to just sell crap on ebay under a new account in order to get funds. Then spend funds on vpn service.
Or you could go to a store, buy a prepaid 25$ visa, register it online with bs info (be sure to use public computer/ free wifi w/ a spoofed mac address if you are serious about being anonymous) then use that to make the vpn purchase. It is recommended not to use your own ip when setting up method to purchase the vpn. If you use your own isp ip address, you CAN be connected to the anonymous vpn service if a court order ever hypothetically came about. They will know which payment method was used to pay for what, and if you are tied in ANY WAY to that payment method, the feds could connect you to anything that you may or may not be doing. If your vpn service will not accept visa, you will have to get funds from ebay or try your luck with a new paypal account. Some new ones won't be "limited" from the start like that. Paypal is funny.
Another option is to make a new paypal, but that requires a new, un abused ip address as well as being cautious of flash cookies or all of your accounts will be linked, and you will be banned from all accounts later down the road at some point. There is a lot of info about going "stealth" on ebay and paypal, look around if you need to.