When it comes to such a price tag, there had best be something worth while. So the question comes down to is he worth while, and can he make his points. Having many games under my belt using him, I've been able to dissect him pretty well, granted though that I do not play many skirmish games of 1500pts or lower (so we are not discussing games that low in points).
He is expensive. Far too expensive in many peoples eyes. I have really yet to see another Dark Eldar player fielding him, yet somehow, he makes every scheduled (non-teaching/non-testing misc things out) game I play. For this many points though, he must really be able to give the army something drastic, and be able to pull his weight on the tabletop.
We all know he gives you the ability to seize the initiative on a 4+. This really gives the enemy player a crap shoot if they decide to go first. If they choose to go first, and you seize, you have the upper hand already; If you go first, that is what you wanted so your happy. So designing the rest of your list to take advantage of this is needed. This alone is great, but that is not alone what we are buying for our 240pts.
He is a monster in close combat. I generally give him 4 Incubi Guard with no upgrades, and throw them on a raider to get them somewhere. What this does, is allow Vect to really concentrate on killing that Independent Character, so that the incubi can take out the rest.
Vect is on initiative 8. Remember that when you take out an independent character on I8 of your close combat round, you immediately gain a pain token. This means that by the time we work down our close combat round, you can often already have your first pain token before the enemy strikes back. Let Vect take out the IC, and the Incubi can get the rest. When you beat the enemy down here, that you are on an I8 when running them down. You will most likely catch them.
Weapon skill 8. You will hit the enemy on a 3+ and get a re-roll. With 7 attacks on the charge hitting on a 3+ with a re-roll, and then always wounding on a 3+, you will kill a lot of things. Not many Characters can stand up to that.
If for some reason though that you do not kill everything that just hit you, you still have your shadowfield for your 2+ save. Now if you lose it, do not forget that our Supreme Overlord wears ghostplate armour, so he will still get a 6+ invul save, or a 4+ normal.
Don't forget you are fleet. This means that Vect and his incubi are extremely mobile, even without a transport.
I haven't even touched on his obsidian orb. When he throws this weapon, it is S10 vs opponents Leadership. 12" assault 1 blast with AP3. His BS is 8. So don't worry so much about scattering. Warning.... Do not use this attack if it will make your opponent make a moral check. However, each wound caused by this will heal Vect a wound. (the model does not have to die to it, you simply have to wound). So if you do two wounds to the enemy, and they make both their cover saves etc... Vect still heals his wounds.
So all this is cool. What has he really done in a game on the table? Ok. Here is my favorite Vect day. With a failed seize roll, his raider got shot down round 1. No matter, he hit the enemy round 2. In this game Vect and his incubi (a total of 5 models) killed 3x 5 man Blood Angel Assault Squads with a power weapon, 2x 5 man Devastator Squads, A Librarian, Corbulo, and a Landspeeder Typhoon. I'll let you add up the points.