As far as I know, you can't, you will need to use a credit card, check, or money order to make the purchase. For more information, contact vBulletin's Sales Dept. directly.
In order to see YOUR specific options, please use the pre-sales forum, (a moderator will probably redirect you to: sales@vbulletin.com). Ask sales for the paypal options for your situation.
It has been said at vBulletin.com many times that the costs will remain the same when vB3 comes out, they do not have any plans (yet) to raise the pricing.