$vboptions[bburl] cannot be used for users who want their wiki in a different directory than their forums, nor for users who have defined any custom wiki URL schemes.
I have fixed the https issue for the next release. Wiki links will be added using whatever the current protocol is. In cases where wiki links become added to the post cache, these links have to use whatever protocol was used when they were cached. However, navbar links and forum-display links should always reflect what's current.
This is an unfortunately necessary tradeoff to making URLs flexible re: protocol. If https is only for paying members, I trust you have a redirect in place for non-premium members who attempt to use it. This way if https links are cached, there's no added benefit for non-paying members. You might also do well to have a redirect for paying members that always directs them to the https version. Of course, you should attempt not to redirect anyone if they are already performing any POST requests.