At the moment I play High Elves as my main army, with several other armies alongside- VC included. A good friend of mine plays DarkElves, so i'm familiar (but not an expert) with the lists.
For playing style, they are all very different from one-another, so it's odd that you're torn between them. I will warn you that all three of them are difficult lists to master, and you're going to get owned a few times before you get the hang of it.
Dark Elves and HighElves are both paper-thin, low-number armies. Their highest toughness generally a 3, DE might have a 4 here or there. Their saves are 5+ on average, so they can't withstand much. However, they are both very elite in different ways.
High Elves have the option to take devastating elite infantry, and can take more Elite choices than any other army. Trouble is, most of their elites are metal and that doesn't look like it's going to change. HighElven magic can shut out most enemies and is on the high-end for destructive capability. Of the 3 armies here, they are the only ones who can take more than just 2lores. Their shooting is also longer range.
The Dark Elves are different from the highelves. The DE style is to get in close to the foe and hack them to bits very quickly. The Dark Elves are the only list you offered that have Cavalry as Core. This means you can make a fully mounted army. Much of the Dark Elf statline focuses on Attacks, Strength, Initiative (although they don't have "always strike first" like HE). They have a darkside appeal, offering brutal, short range shooting and dangerous offensive magic. They are high-end magic, but don't have the same shut-out power that the HighElves have. Dark Elves are actually the most like their WH40K kin, the Dark Eldar. Both are short-range, high-impact, fast armies.
The VC we currently have are pretty good. I'm not sure what the new book has in store. Either way- VC are a SLOW army. They offer you the ability to have A BUNCH of meatshield soldiers on the field. Most of their statlines are very low, zombies, skeletons, and the likes are just HORRIBLE fighters. But they never run away, they actually Scare their enemies, and a Vampire is one of the best fighters in the game. Their magic is dangerous, if only because it lets them raise MORE shoddy fighters. Opponents are often left slugging it out with MASSIVE units of undead that just refuse to stay down, while lone vampires stalk from combat to combat slaughtering tons of troops. The playstyle can go a little stale after a while, basically: you put out your guys, and I'm going to flood the field with mine. Now i move towards you, you have no target but my big units. Ressurect more guys, fight a hopeless combat against your guys, then charge my vampire in to waste you. It's essentially like using a gaurd army, without ANY shooting.
The choice in models and playing style is up to you, remember, balanced High Elves, Aggressive Dark Elves, or Steam-Roller Vampire Counts.
Hope this helps.
.
