Is there any particular reason /why/ Vallaki is the starting area? Besides of course that Barovia in the books is like the "beginning" and tabled for levels 1-10? If not, I don't see a problem with randomly dropping players in different area's, as long as they're able to get a different intro, and also making all of these area's accessible to both high and low levels.
Keep the XP system in place where you get less xp for higher levels hanging around. This is that stops them from just carrying lower leveled PC's everywhere, unless it's for raw information, which is okay.
When delivering a threat, figure out how exactly you want to deliver it. Vampires can be as strong as PC's, there are Vampires, or rather wonders, just as strong as any other PC in the game, this can be a "Crippling Horror". Say a red mist or whatever saunters through and destroys anything it touches. It's also unkillable, this checks even High Levels. If you're focusing on delivering something to low levels, just follow them around for a while and give them a plot to tangle with. Don't make it so that they think it'll be hard or impossible to do.
Make them THINK it will be easy, then flip them on their head when they least expect it, in a place far from high levels, and make it a challenging experience without killing them. If a high level interfere's, just ramp it up on the high-level, and depending on how they're interfering, perhaps just outright kill them.
There are so many plots that even the playing field, even take simple ones from the first Ravenloft books. You can't kill Strahd without that magical sword right? Well you could just make that idea extreme, what's stopping you from running an adventure that makes it so that a certain scroll has to be acquired to make a vampire or Witch vulnerable?(mages) Or a certain sword? (fighters) or a certain holy symbol? (clerics) or a certain pair of keys?(Thieves) You can make everyone feel special and important, and this is something /EVERYONE/ can particpate in, regardless of level, because even your level 20 Cleric of Torm can't take on my Level 13 Vampire, because of whatever favor he's gained from the Dark Powers. The answer is one that can be answered by anybody, any group.
In this way, we can intermingle these groups.
I totally agree it's not the DM's "responsibility" to remedy these problems, and the fix for making people feel like they're not always protected is one that requires a DM present in the first place. However the situation that exists is usually presented by a DM anyways. I see people complaining about how DM's can't scare the players because high levels are always around. Well, the DM's already causing that problem in the first place, so they also have the ability to fix it.
As far as wandering at night, the developers have done an astonishing job of making Werewolves fast, deadly, and smart. I think this combination usually leaves low levels feeling pretty scared, even if they're traveling with a couple level 12's or whatever at night. Furthermore, a single swipe from some creatures is enough to bring down level 5's and such, depending on the creature, so they still have fear components.
In short, I don't see why we have to separate level 20's from level 2's.