Currently you can only scroll the whole page. If you have a big folderstructure that sometimes means scrolling down the folders just to have to scroll up again to view the passwords. If the filtercolumn would have it’s own scrollbar the problem would be avoided.
The web-vault and other clients are developed by the upstream project by the Bitwarden team. Vaultwarden only pre-packages the web-vault client with the back-end Vaultwarden server package, along with some custom tweaks to remove some elements.
For any major changes such as this to the front-end client I would recommend suggesting this feature request within the official Bitwarden community forums