Hello! I've been working with vbulletin software for 10+ years and I'm confident I can modify things for you to your liking. I'm in the US (near philly) so I could even talk on the phone if you want.
Three of your requirements can be done with the stock vbulletin subscription system, usergroups, and conditionals in templates. The replies per page will require some modification, which is within my abilities. The payments can be handled through Paypal and 2co, which I'm familiar with. If anyone bids on this saying you need a custom system to handle everything then you're probably wasting your time (and money) and they haven't worked with vbulletin that much in the past.
By the way, I'm currently a little new to working on Freelancer, but you can check out some of my other work on [login to view URL], [login to view URL], and [login to view URL] . These are all sites that I own and have been heavily modified either through templates, available modifications, or custom modifications (some completed by me and some through outside contractors. For example, I did not code the Phone system on [login to view URL]).
Please let me know if you have any questions!