I suppose, that something like teleporting 1000000 space marines to Comorragh or ramming gloriana class battleship in it could be part of emperors plans during great crusade.
But current imperium is too busy not falling apart for doing anything grand.
Ok, so one million Space Marines and one Gloriana. What else? Do the guardsmen need to be in the billions or the trillions? How many Emperor titans? How many battleships?
Commorragh has some nigh unbeatable defenses. For one thing just getting there is very hard since it requires passage through a webway portal, most of which are in Dark Eldar hands, and which few other than the Eldar can activate.
Secondly almost every citizen of the dark city is a soldier on par with Imperial Guard veterans. Not all are well equipped, but this is a city fight in a battlefield most Imperial vehicles can't even enter, let alone move with any speed.
Thirdly the city can be divided by nearly impenetrable walls of energy, all controlled from and (presumably) powered from Vect's fortress. These walls can completely contain Orkish Waaaghs!!! and Daemonic Incursions. Very little among the Imperial arsenal poses a threat to them.
Fourthly Vect can, at will with only a small delay, expose any part of the city he wants to the focused energies of all the stars that are used to light and power the city. This can melt entire shielded fortresses in moments. It could destroy titans, battleships, and pretty much anything else that can't survive point blank contact with the Sun.
Fifthly, a lot of Commorragh is spread out in little pockets and islands in the webway very far from the main city. These subrealms are reached through portals that seamlessly bridge the distance between them and the city. If the Dark Eldar thought they were losing a fight they could just shut them off and hide in the sub realms for a few decades until the Imperials leave. Only a few kabals would survive, but that's all they really need to continue their kind.
The Dark Eldar only number a few tens/hundreds of Billion though, so it's not difficult to wipe them out if you really give it a go. I don't think the Imperium in it's current state could do it at all though. Too many different threats to take on. The amount of military might it takes to fight an Eldar Craftworld and win is generally multiple chapters of marines and a big chunk of a Segmentum fleet, and Commorragh is much better defended and more populous than any Craftworld. Trying to scrape up the manpower to wipe out the DE would lead to so many parts of the Imperium being left undefended that it would be a net loss for mankind.
What force would be needed to raid Commorragh and retrieve Panacea? How many soldiers? How many battleships? How many chapters of Space Marines?
The Panacea STC was probably destroyed by Vect, or by his Haemonculi. It's of no use to the DE themselves, they already have something better. Considering it was taken as a prize by Lady Malys as a way to prove herself to Vect it's possible he even destroyed it as part of casting her out in a display of contempt.