^That's right; Quirinal was repaired on the surface of a planet.
Exactly. If memory serves, CotS states in the Appendice somewhere that Achilles was just over 900 meters or so... if I recall correctly. With a Vesta about the same length as a Sovereign, the Achilles would need to be over 1500 meters if it could not only fit in a Vesta, but also house crew and equipment needed to service and repair a ship. Not to mention everything needed just to operate and maintain the Achilles herself, like warpdrive, fusion reactors, crewquarters, stuff like that....
Just checked on Memory Beta: http://memory-beta.wikia.com/wiki/Mulciber_class
960 meters long. There's no way the ship is big enough to house a Vesta class inside.