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:
cache directory..htaccess file was also empty, so I copied the one from vanilla/vanilla/blob/master/.htaccess.dist. If you have a $Configuration['Garden']['RewriteUrls'] line in conf/config.php just delete it.$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.[forum url]/utility/update, instead go to [forum url]index.php?p=/utility/updatecontainer.htmlplugins/EmojiExtender (there should be a new dir call plugins/emojiextender)themes directory has moved to addons/themes.$Configuration['Debug'] = true; in your configBut I ended up starting the forum over because the database was a big mess, and would take too much manual intervention to get working right.
Other notes
☞ If you want to add or remove themes, or generally mess with the plugins/themes directory, you might 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.