Sometimes, depending on the forum, a VIP can have a lot more priviledges than a regular user can, besides posting in a special forum. The most I've heard of is more PM space, more advertisement space if it's an advertisement forum, etc. It's all a matter of what the forum si about, as to what you can offer with it.
You need a reason for them to donate. A 'rank' doesn't seem to be enough to get most people to donate, IMO. You would need to offer something more, like exclusive stuff (like maybe a rare item if you ran an RP forum).
To be fair Digital Point has 13 million posts and nearly 2,000 online users right now. It's also targeted at webmasters with much larger websites, so to them $125 annually really isn't a lot of money and is well worth the added exposure.