Vandalism and Spam
For questions and reports about this you can go to the VSTF Wiki. You can find there several pages where you can report vandalism and spam.
Bad admins and other bad situation on a wiki
For this you'd best contact the admins on the wiki you have the problem. If an admin is doing bad things in your eyes you'd best contact the admin by yourself or contact another admin or bureaucrat on the wiki who can talk with the admin, but a bureaucrat can also take away the rights. If your're blocked and can't leave a message there you can do that on Community Central. If the admin also has bureaucrat rights, you can't do that much. The only way to solve things is to contact the admin by yourself. Or you have to organize a community consensus.
If you want to know the reason why you or somebody is blocked you can ask this via Special:Contact/blocked. Wikia staff will answer your question there.
Blocked or Banned
If you're blocked or banned the best way to ask why is going to the message wall/talk page of the admin or chat moderator who blocked or banned you. If you can't leave messages you can leave a message on Community Central. Don't forget to check first the block log, you can usually find the reason of your block in the summary.
If you want to close/rename your wiki you can request this via Special:Contact. If a wiki is closed and you want to know the reason you can also use that link. Please remember Wikia doesn't just close a wiki because one user wants that.
Issue with the content of a wiki
You can use Special:Contact/content-issue to report this to Wikia staff.
You can do this aswell on the page Special:Contact. Remember you can only change your username once, so choose a name you really like.
Problems with Account
Also for questions about this you can use Special:Contact.
Become an admin on a Wiki
If no admin edited for 60 days you can do an adoption. But you can also just ask a bureaucrat on his/her wall to make you an admin. On this wiki Wikia staff makes people admin or chat moderator if they want.
Technical and admin help
Normally asking this on the forums on Community Central is a good place to ask it. Of course there are numerous other places where you can ask for these help.
If you found a bug you can report this via Special:Contact/bug. Often Wikia staff wants also a photo of it, so don't forget to take a screenshot for example.
If you miss things in this blog post let me know in the comments below. If you want to contact me you can go to my message wall.