The sparks are going to spawn from either the NW, NE, SW or SW "corners" of the room. I use corners loosely, since the room is a circle...but you get what i mean.
You should have 1 Dedicated person looking for sparks You want the sparks to be coming towards his ***. So if you are tanking on the north side and a spark spawns from SW or SE you can keep him where he is. If it spawns from NW or NE you have to slowly move him in a huge circle so that you are tanking him on the S end of the room. Its a bit tricky to do at first, and healers have to be on top of you incase you go out of range, but after a few tries you should be able to do it fine.
When you land from vortex, malygos will not breath for a few seconds. Everyone in the raid lands in the center of the platform. So before vortex, the person i mentioned before that is calling out where the sparks are coming from needs to watch, there will always be a spark spawned right before a vortex cast. So if the spark spawns at the NE corner. When everyone lands from vortex, you entire raid runs NE 5-10 yards, and you take malygos to the opposite corner (in this case it would be SW). This will happen every Vortex landing.
So whether you have to move him is solely dependant on your luck, you could get sparks from the same general direction (NE or NW) or (SW or SE) and not really have to move him at all.
Its not required to have a DK DG the sparks, infact if your DK can save his CD for when a Spark spawns in a bad place (if you are tanking him facing NW and it spawns NE) rather then move him, you can have the DK DG it into the middle. Ive done maly several times with no DKs at all, its 100% doable, just slightly more challenging.
If i missed anything, i apologize, i am only the Holy Paladin :-p. But i think i covered most of it, and answered your questions. Good Luck.