I've got my perfect layout already (older pic, same layout though), all rooms level 3, max caps, and it takes 114 dwellers to man it all (plus the 4 legendaries I've found). Even though I only have 118 dwellers, I have the living quarters and resources to easily support 200. Those 118 all have max SPECIAL and are level 50, all leveled from 1-50 with 15 endurance (or whatever they started it with the legendaries), except my 10 explorers which did 1-50 with 17 endurance. In those 118 I even have 18 dwellers (15 women and 3 men) exclusively for the purpose of grinding pregnancy related objectives.

Having an average level of 50 does make deathclaws as tough as possible, but it's no problem for my kill room. My dwellers up there all have legendary weapons I've found from lunchboxes, ranging from a rusty fat man (22) to an accelerated gauss rifle (16-20). Deathclaws always die in their second pass through my kill room, and with 595 health each, my dwellers are left close to death, though they never die. I can actually leave my vault alone for the duration of the deathclaws and not worry at all, just need heal up after.

My 10 explorers each have a pressurized flamer (15-19, the strongest weapon you can find in the wasteland in terms of auto-equip), as well as a legendary outfit that provides at least +1E for radiation immunity (except one explorer who has a rare cause I don't have a legendary for her). With 25 stims all of my explorers consistently reach the auto-return mark of 100 items, so I never have to worry about them dying.

I'm currently at 4 Mr. Handys, and I just need 2 more to fully automate collection from all of my resource rooms. Of my 118 dwellers, the worst weapon I have equipped on any of them right now is a rusty gauss rifle (16). Ideally I'd have them all with a plasma rifle (17-18) at worst, but that would take quite a while and a minimum damage of 16 is pretty damn good. All of my dwellers are in +5 outfits based on the stat of the room they're in, except for my power workers. All I need is 23 more +5S outfits then I'm good (I've been rather unlucky with finding them). The ones I do have are on all my female workers, so the ones without for now can wear the sports fan outfit for +4S and +1L.

I haven't paid a cent on the game, all the Mr. Handys and legendaries I've gotten are from free objectives. I can grind a lot of objectives for the most part, so I think realistically my end game is when I find the last of my +5S outfits and get 2 more Mr. Handys. It's as perfect a vault as I can get realistically.