I found a fix for this bug using the console to reset the quest line:
1. Save your game just in case something goes wrong.
2. Open the console using the tilde "~" key.
3. Type
resetquest 0015D79D (those are zeros, not the letter O)
4. Now that the "Still in the Dark" quest has been reset, speak with Elder McNamara to get the quest again.
5. Thankfully, all your work from before still counts, so just keep talking to him to complete the quest steps. Eventually you will need to go talk to the other NPC about the parts you retrieved from the vaults. Do this, then head back to McNamara.
6. After talking to McNamara, the quest will be complete.
*Some important notes:
I didn't talk to Hardin at all - I didn't want to risk hitting the quest bug again.
You will receive XP and reputation again - I consider this a bonus for having to go through this crap to fix a buggy quest

Here's where I got the quest ID: http://fallout.wikia.com/wiki/Fallout:_New_Vegas_quests
Hope that helps.