For the most part, you have the frame down. However, you are running Vileplume, so the need for Rare Candy is gone.
-2 Rare candy.
That being said, you obviously want a full evolution line for your Gengar, so add another Haunter, and a Gengar level X.
+2 cards.
Seeker isn't really needed, but if you want to keep one, that works out fine.
-1 Seeker.
You want more shuffle cards, like Lookers.
+1 Lookers
As for the remaining 20 cards, try adding more Supporters(Collectors and Bebes first and foremost! Ups consistency, and they are just staple supporters). Add an Azelf, in case your X or something valuable gets prized.
Unown Q is another good option, as it can get your Spiritomb or an Uxie out with free retreat. A healing pokemon could be good, like Nidoqueen or Blissey(PT). Blissey works pretty well in the mirror, because you can discard supporters, heal, and that makes them do less damage(if they are even attacking your Gengars with poltergeist).
Add in a couple of Warp energies(3 is good), in case your Vileplume gets pulled active by a Luxray, or Blaziken. Don't let them stall you! Also, a 6th psychic wouldn't hurt anyone

.
Try adding one more kind of shuffle card, like PONT. Three of these in this deck are really good, because you can dump your hand of evolutions, and get a fresh six. Also, it helps in the case of bad starts.
Hope these tips helped!