I have, I assure you. Huge, take up most of the map. Typically have a 'core' area that contains fairly large amounts of gold on the floor, and between 1-4 artifacts on the floor (usually, in the corners of said 'core' area). Outer areas has oodles of monsters of a particular type, with the center area having a few more around the inner edge (usually) with much tougher varieties in the very center.
If magic mapped, most also typically (for whatever reason, I don't know) either have a single door (literally) in leading in. Those that happen to have multiple entrances to the vault, are typically (though blatantly obviously) masked by hidden doors. This can be be used as a major advantage, should you for whatever reason not want to challenge the vault, as the large number of monsters within the vault ALMOST pacifies the dungeon level in question. You may run in to a couple of creatures, occasionally, that aren't all locked in the vault with the goodies.
In thus, characters wanting to achieve an ultimate/ultra ending, if they're lucky enough to get a few of them. - I've noticed almost every character that gets down that low, gets at least one. A couple don't, but, are often made up for with the 'surges of power' in another location. The rare cases (which are extreme, at least from my experience) that get more than one, dispite getting get less 'surge of power' artifacts (if any at all), and, also tend to get less artifacts from each greater vault they encounter.
The most artifacts I've ever had generated on a character at one time, has been 42, with an assured 44 being the top off on what I 'knew' I could get (as I completely avoided gremlin cave and quickling tree). Anything further, would have to come from pick pockets for a very small number of pickpocketable artifacts, and random generation (likely easiest from the blue dragon lair, as if I'm not mistaken the varieties of dragons, save for the 'baby' types, have the highest chance for an artifact generation on death). Could raise that balance to 45, if you count the shield in dwarftown that's 'available' for lawfuls. 'Available', but not assured. I've read that on several occasions he didn't give a lawful the sword. Probably linked with the altar, in that receiving the shield is based on your alignment when you FIRST enter dwarftown (i.e. how the altar is lawful if you first enter dwarftown as a lawful, and neutral if you first enter as either neutral or chaotic), which also affects how much you can 'buy off' your alignment by giving the dwarf priest gold (MUCH faster than using the '1 gold' method, and, zero risk of item destruction, altar conversion, cursing, getting zapped, or having creatures summoned with intent to kill you. If you enter first as lawful, 'buying' your alignment up, can be raised to 'L', while, if you first enter as neutral or chaotic, this method only works to 'N-' (which is more than sufficient to get ToRR, if that's your only reasoning for alignment change in the first place). Thus leading me to believe that all of dwarftown is heavily alignment based, on your first entry (similar to how you have a single 'fated' artifact, that dispite how hard you try, will always be your crowning gift for that character.
I also believe that vault (or at least greater vault) generation is heavily influenced by the number of artifacts that you've already found, and/or have been (pre/post)crowned with. A character with only about 10 artifacts generated (a hard thing to do, beings as even if you didn't pick it up, it still was generated) will have a much higher chance for a greater vault, or surge of power, than someone who already has 20-30+ artifacts.
As far as artifacts and religion go, the most you can receive from piety is 16 artifacts - 15 precrowned (8, 11, 14, 17, 20, 23, 26, 29, 32, 35, 38, 41, 44, 47, and 50), and 1 crowned. Doing this, however, will also (usually) make you sacrificed a large number of artifacts that have no 'random' generation (ankh, AMW, minotaur axe, ect) that are based on level. This can be 'worked around', if you have a huge amount of spare time, the gardening skill (stoma's), select the black druid quest (but never enter the level he's on), and have been lucky enough to have somehow garnered a bunch of danger scrolls (you'll also, obviously, need a way to pacify a map, likely with an altar really close to it, that's also near the surface - PC: 1 is a good place, if it has an altar spawn). Most of you can probably catch my drift with this, but, getting the 15th (and probably 14th, for that matter) is almost completely theoretical, as piety will decrease with time. So, while your desperately trying to powerlevel yourself in a danger scroll augmented pyramid (not fun), your piety is constantly going back down. Though, even IF you happen to be successful in getting all the way to level 50, and managing to nab a 15th precrown BEFORE you ever enter the top floor of the pyramid, manage to get the ankh and ancient mummy wrapping, and survive all the crippling corruptions you likely have picked up by now (as you'll likely be gardening your way well past day 90, just to get this started)...you still lose out on the minotaur axe.
After that, you can still go pick up the torc, Keethrax will still be there. However, even killing him (and even giving the corpse to the druid in town) will not give credit for completing the quest. The druid will simply give you the unicorn quest start, and not mention about Keethrax ever again.
What does all that have to do with greater vaults? Mostly nothing... As, if you manage to complete even over 2/3rds of it, you'll more than likely be extremely lucky to even see one greater vault, and, even if you do...it'll (almost assuredly) only have one artifact in it. So, if you do something whacky like that, you can be pretty sure you wont get many 'vault problems', unless they're of the 'lesser' or 'royal' variety. That's what.