I thought vampires were to dress to blend in with people?

Why are they wearing the same thing? It's like a uniform they have going on. Like they're all dressing to impress Molag Bal. I noticed only a few dress like the towns folk or in black robes. But seriously?

Red,white, or black vampire armor? No other armor?

..and why do they use the same old attacks over and over? Like the vampire drain and destruction magic? Sure, you get one with a sword or ax in one hand, and than drain in the other. That gets really old. I think they need to change it up a bit.

In Oblivion, vampires took any class. In this game. Vampires are in their own class which is light armor, destruction, one handed, and who knows what else aside from Harkon and Serana( Serana not being that much different).

Who's "they"? Bethesda won't. The vampires won't do it themselves.....

Kinda answered your own question right at the end there. They've become their own separate class, meaning copy and paste free for all.

Dawnguard added the vampire armor and changed them from having regular light armor or robes to having that.

It gives them a unique aesthetic. If I recall correctly, people complained that fighting vampires didn't feel any different from fighting bandits in Oblivion or Morrowind. They had a unique absorb health spell in Morrowind, true, but Skyrim's spell is visually different from run-of-the-mill absorption spells.

Also remember that the vampires who wear their distinctive armor aren't in cities. Vampires who are trying to blend in wear normal clothes and armor (for example, Sybille Stentor).

They only dress up in Vampire gear when they're attacking towns openly and aren't trying to blend in. Vampires like the ones you hunt down in Hide and Seek and A Jarl's Justice dress and act like ordinary citizens.

And their attacks do vary as well. They often resurrect "helpers" to fight for them. In an attack on Falkreath the other day, I saw them kill, and then resurrect, the town goat. Didn't help them much.

There's quite a few vampires hidden among the common rabble, and most of the time you can't tell at all from their outfits or attacks until you find that vampire dust in their inventory. All the major factions in the game seem to wear their signature armor or outfit most, if not all of the time (except the companions), although if you're a long distance archer I'm not sure that's such a bad thing. I don't know how many times I've spotted Farkas out in the wilds from a distance and stuck an arrow in him, only to have to reload because he wasn't a bandit. There's 2 sides to that particular issue. I do agree that the stock vampire templates needed more diversity in combat styles though. As a predatory race, the last thing you want to be is predictable.