Ahem. Now that I've actually played the DLC to its peak, I think I can say why no one should by it, and maybe even should try to get Bethesda to give US money for subjecting us to this trash.
First and foremost, you don't even build a god dammed vault at all whatsoever. You build a settlement with a vault theme, that happens to be underground with a big vault door. The most you're going to get from this being a vault is some settlers going "oh weeeeewwwww, I'm a vault dweller. kewl." every now and then. They're not even renamed to Vault Resident or some shit. It's just a settlement with vault themes and a more obvious line on where you can and cannot build.
Speaking of which, You get pretty much FOUR whole places to build big stuff with a lot of stuff in them. I think you can even connect them all if you can handle the terrible controls. BUT! The build limit has not changed at all! I dunno if it changes in the other areas, but by the end of today I had like a quarter of building limits left.
Also, the quest is terrible. You build four things. FOUR THINGS! And then done. The coolest part, which wasn't even that cool, was where I had to find a buncha gunners choking on Joker Gas and killing each other. That was pretty neat, but it left as fast as it appeared. Other than that, build a thing, let Clem go "WEW! THIS IS KEWL! OW! IT HURT!" and then move onto the next until ghoul lady leaves.
Also, how is lady a ghoul? How did that happen? Aren't vaults meant to protect people from the nuclear blasts? Why is she not a dead regular human? And why is she not feral? Everyone else is feral? Why is she not insane? She's pre-war. She's been stuck in a single room with no one to talk to for god knows how long. And to top it all of, it's the fridge kid again and she has had nothing to eat or drink the entire time. I mean, sure there's a can of cram on a fire barrel, but I highly doubt that a buncha cans of cram could help feed her, and supposedly more than one, or even up to five people for OVER TWO HUNDRED YEARS!!!
In summrary, this DLC is a pile of trash, I don't like it. You shouldn't like it. You shouldn't buy it. No one should buy this willingly.