Vault 45 is located on an island in the middle of The Mississippi River between Iowa and Illinois. It was constructed beneath The Rock Island Arsenal, a military base known for its production of munitions. In fact, it is the largest producer of weaponry in the United States.
The exact location and entrance into Vault 45 was kept a government secret, and even those residing within the vault have no idea where it is.
Before the Bombs…
As more conflict swept through the later 21st century world, The Arsenal ramped up its productions, including being the site that produced the very first Power Armor suits.