While the "minimum level" marking is useful, there is still a ton of variance in difficulty for the different areas.

The two starter levels are very much survivable, followed by the slightly more difficult Sunwall (which is much easier if you skip the vaults).

After that, it gets a bit chaotic. Ritch Hive and Ruins are both a bit hard, while the Dominion Port is easier, and the Whitehoof subquest is also easier, though the boss is somewhat challenging. The Angry Shopkeeper quest is hilariously undersold - you can peacefully shop at your leisure, but it is extremely dangerous to open hostilities before level 30.

That's about as far as I've gotten. It seems like having enough discipline to skip some vaults (but which ones?) and doing the zones in the right order is as vital as your build and strategy, much like most of the early-to-mid game of the original campaign.

Anyway, has anyone figured out a zone order or found one online yet?

I find best order is Yeti Cave (if an orc), Emporium (reverse if either of the other races that start in the Emporium), Sunwall, Ritch Hive, Dominion Port, Krimbul Territory, (Maybe the ruins here, but I've encountered some pretty tough enemies there so I've been leaving it for later so far), Observatory and then the Internment Camp, which is as far as I've got without dying.

The reason for that order is largely that although the Ritch Hive can be pretty tough, it's mostly manageable until the last floor. Whereas the Dominion Port I find it hard to resist opening the culverts and some of the boss alligators that come out can be pretty tough without a few extra levels under your belt..

I've been avoiding the Hidden Vault levels in both entirely as so far all my attempts to enter them have led to swift death at the hands of way over levelled enemies.

I've given up on perma-death. I'm just doing Explorer now so I can finish. I think you need to clear out vaults for XP, but to do so safely, I bet you'd have to backtrack a lot. Which would be pretty annoying, and it's hard to resist opening all of the things.

I'm kind of tempted too.. mainly because the lack of races and (fun) classes available is a little short and I've basically ended up playing the same Yeti Sawbutcher over and over again..
But I don't think it would feel the same if I didn't have permadeath..

There's vaults and there's vaults... the ones in the Sunwall for example are usually alright (though I was caught out and killed once by a tough mage type thing in one) but the Vault levels (the blue stairs called Hidden Vaults) have so far proved impossible every time I try them.. and the vault I found in the Ruins contained a lvl 40 or 50 elder dragon (or whatever they call them) when I was lvl 20 (or even less), which unsurprisingly slaughtered me.
In my winning run in the first campaign I basically ignored almost all of the vaults (the ones in Daikara and Dreadfall are the worst, pretty much impossible) until late in the game where I was nigh on invincible.

What annoys me is that the Dominion can't be entered again once you complete it so when I find a vault there I'm really tempted to visit it (same reason I always end up opening the culverts), at least you can come back for the Ritch Hive ones...