1. REQUIREMENTS LINK :
https://docs.vanillaforums.com/developer/installation/self-hosting/#requirements
2. We have added $Configuration['Plugins']['GettingStarted']['Discussion'] = '1'; in config.php because it requires writable permission on creating discussion for the first time. Check and add this if we need to add this in next version.(NOT Required now)
3. We have removed "build" directory from original package of vanilla 2.5.4 because it contains "vanilla 2.6.x vendor" directory. (Since 2.6.1)
4. We have removed $Configuration['Garden']['InstallationID'] & $Configuration['Garden']['InstallationSecret'] from config.php. It creates after installation Settings->Statistics.
5. We have kept min required version to upgrade as 2.3.0 because on versions < 2.3.0 password method was different and password backward compatibility is not handled by vendor in 2.6.1 package. Hence after completing upgrade successfully, we can not login into admin panel.