[hambil]

Well, you can only customize existing vbulletin access denied messages. The closest you can get is to make the forum so they can view it, but view nothing else (can't view thread contents or create new threads, etc.). Then use this tool to create a custom message for that forum and usergroup(s).

This to me actually seems like a better solution anyway, because the non-donating members are teased with the thread titles, and more encouraged to donate when they try to read and thread and get the message.