Oh boy I love vampires.
Why the vampire has been asleep for so many centuries is purely up to you and you can do whatever you want. That's the true answer to all these questions- your story, your rules. In the Sims 4 specifically, vampires can hibernate for as long as you want them to. It keeps them out of the sun and they're basically in torpor until you take them out of it. Your guy could also be trying to wait out something for 200 years but the recovering thing also makes sense because *if I'm remembering correctly* their needs do not go down at all in hibernation.
About paleness, super up to you. I know in Sims 3 if you took a not super pale like Vlad sim and turned them into a vampire they'd get a grey cast on their skin, but there's nothing like that in Sims 4. Story wise you can fully ignore the paleness or maybe have your vampires use cosmetics to cover their greyness up. A show I watched recently that's kinda vampire-like did the second option, combined with full special effects makeup.
In Sims 4 a vampire can become human again. In your story I would just make sure not to have it happen out-of-the-blue. Build it up, make it important. Most of the time becoming a vampire is permanent but it's fully up to your discretion. Maybe it's a certain ritual, or a magic item, or it requires training and a quest. Maybe it's impossible, but your vampire doesn't know that so they try anyway.
In game the cure is a drink you learn how to make with a really high Vampire Lore skill. I don't remember the level, but it's made with a mixology station. The level of mixology your sim has doesn't matter.
If you have anymore questions please ask I'd love to help!
Oh, reading over your post again, perhaps it was impossible to become human again before but with new advancements in technology other vampires have found a cure. And perhaps even some vampires think this is horrible and vampires should stay vampires. That's fully up to you though I'm just giving ideas as I get them.