I went through this to add NZD - New Zealand Dollar. I've got this working and tested A-OK with PayPal. I haven't bothered with any other of the built in gateways. Keep in mind that with every upgrade to vBulletin you'll have to edit the new files. The write up I made was here:
https://vborg.vbsupport.ru/showthread.php?t=232290
I have no idea why it's hard coded like this. Hopefully in the future there is some work done on the Subscription System.