In vBulletin, one can not make contribution/donation using payments.php as anonymous. What if someone wants to make a anonymous contribution to the website and may not want to register? Many people have shied away from registering and donating just because there is no inbuilt anonymous method to make a donation.
PayPal handles recurring payments. All a subscription system does is automate changes to a person's user groups based on what they buy. If you don't use that, you can get HTML/buttons from PayPal to allow people to pay you.
Though I have to ask, if someone is dedicated enough to your site such that they'd donate, wouldn't they have registered anyway? It's unlikely that one-time visitors will send you any money.