You Can Build Vaults in Fallout 4 Without the DLC
If you want to start tinkering with vaults without Vault-Tec Workshop, you should know that there's an array of Fallout 4 mods focused on this exact thing.
We recommend taking a look at those mods if you aren't sold on the DLC. There are a number of excellent ones out there that will only improve as developers push out updates.
Who knows, maybe you find one you like and save yourself $5.