Buy a big hole in the ground

The Vault-Tec Workshop DLC is structurally simple: it adds an incomplete vault, which you can build out like any other workshop/settlement.

If you loved Fallout 4's base-building, then this is a big, open opportunity to flex your creative muscles. As for me, I kinda hated the workshop's janky UI and laborious inventory management already -- so a bigger sandbox does nothing for me. I was done building my Vault 88 pretty quickly, since I didn't use any floors, or walls, or doors; and I'm not at all ashamed about it being the shittiest vault in the Commonwealth.

I did, at least, glean some satisfaction from the small amount of questing accompanying the vault. At first, you have to clean out its rodent and ghoul and radscorpion problems; and the workshop creates some fun opportunities to fight monsters with defensive sentry guns. I enjoyed those moments more than I probably should have.

Then, when the vault is ready for settlers, your overseer will direct you to run behavioral experiments on them. These experiments are small, and completely scripted, but do show off a bit of Fallout's sense for dark humor.

Barring whatever you build in it, there really isn't much to this DLC. At least it's not as much of a pain in the ass as the Mechanist was.

Better than: Fallout 4: Automatron
Not as good as: The Elder Scrolls V: Skyrim - Hearthfire
I guess some people get really into the workshop thing: I'm just not one of them.