I thought vault 34 was very hard compared to the others, but you do get some great loot out of it. The All-American is awesome.
^ ^ This. Most difficult, yes. Most annoying, with constant radiation and a confusing series of puzzles to open key doors with, definitely. Most rewarding with awesome loot that makes it worth going back a 2nd, 3rd, 4th or 44th time? Completely. Once you've cleared out the other vaults and experienced their stories, there's no real reason to ever go back again, other than an odd skillbook or quest item or something. Most of their rewards are completely underwhelming. But the Vault 34 armory? That's worth learning and perfecting the ability to clear the place efficiently, which I have.
After 2-3 times, it was no problem at all. You take your advanced rad suit and plenty of radaway and rad-X, the radiation is a non-issue. I come back out with more of those meds than I go in with usually, due to there being plenty down there to loot. Using stealth and keeping my companions safely stashed well behind me where they don't cause any trouble, it's downright fun to stalk the feral ghouls all through the vault- and once you learn to expect several guard ghouls to regularly spawn behind you and are always ready for that, you rule the place.
Loot-wise, the best of the other vaults might rate a 3/10. Vault 34? Easily 11/10. Only vault worth bothering with.