I am in Vault 22 and have found the keycard but the locked door to the cave on level 3 wont open. Any ideas on how to get to the HEPA cartridges to complete the Still In The Dark Quest.
Any suggestions on what to do?

Thanks for help

Is that the vault with the plants? If so, to open that door you need to use a terminal in the Overseer's Office and unlock it from there.

You need to go to Level 5, go outside into the caves and climb back up through the caves to get inside the ruined room on Level 2 that was inaccessible from your initial position. The HEPA filters are in a locker in there.