I'm in the process of upgrading an old installation of the Vanilla forum software, from version 3.3, to 2021.009.

Still running into some hiccups, but this's what's helped so far:

• I had to clear the cache directory.
• I had trouble with the 'pretty urls', as they've since become the default. My .htaccess file was also empty, so I copied the one from vanilla/vanilla/blob/master/.htaccess.dist. If you have a $Configuration['Garden']['RewriteUrls'] line in conf/config.php just delete it.
  • If you can't enable mod-rewrite then add $Configuration['Garden']['RewriteUrls'] = false; to conf/config.php. The links in the header will have to be fixed manually somehow, but I think the rest will work.
    • To finish the upgrade it will try to send you to [forum url]/utility/update, instead go to [forum url]index.php?p=/utility/update
• Apart from the cache, there's some other stuff that can be deleted:
  • container.html
  • plugins/EmojiExtender (there should be a new dir call plugins/emojiextender)
• themes directory has moved to addon/themes, and my old themes don't work.

☞ If you want to add or remove themes, or generally mess with the plugins/themes directory, you'll have to delete the related stuff from the cache. Maybe there's an easier way, but I've been SSHing into my cache directory and removing (rm -r theme/ theme-index.php) theme-index.php and theme/. Sometimes it'll prompt you to do a refresh in the UI, but often I'll just get an obnoxious error page and won't be able to get into the theme settings.

My old theme doesn't work. It seems they've got much more complicated, so I'm currently experimenting to see how uncomplicated I can make one. I have no desire to fuck with building something as simple as a theme. Kinda wishing I'd given up on the old forum and switch to PHPBB.
Oh fuck, my old theme does work! It just need some fixing-up. I guess I didn't clear the cache properly before.