GRRRRR!
So, you share the area with the 3485769375 other players doing this atm, and the npc's will disappear the moment anyone else clicks on them...
Seems you really should group up for this one, so that you don't waste your daily shot at extra [currency=1755].
My sanity ran out at 6/8 complete (doing solo... urgh... )... hopefully the quest stays in my log and can be completed tomorrow alongside tomorrow's quest (will update here tomorrow if I can or not)
UPDATE:
So, I've come back today with hope... and unfortunately it seems the quest gets removed from your log during the daily reset... so if you derp it up in your 1 try each day, that's it, you've failed. Real shame that... :(
UPDATE2:
Ok... been a while since this came out and me posting this.
The above is still true, EXCEPT, it seems if it's the first lesser vision quest of the week (and grants you 3k coalescing).
Note this affects any of the lesser vision quests, not just hand in the dark (so also the 'close portals' 'kill 20 things' and 'do any of the above to fill a bar' quests)
From a couple times where I've derped on a lesser vision (the uldum/vale vision) on an undergeared alt, and it was it's first time there, the next day the quest has actually stayed in the log, and was completable alongside the next day's quest (the 2nd day's quest will initially say it grants 3k too, same as the first, but after handing in one of them, the 2nd will revert to the usual 1.5k reward).
Shame this doesn't happen normally... :\ or that there was a way to 'buy in' for a 2nd attempt.