And...?

Let me ask again: Which vault(s) are currently, or at least in recent enough memory for anyone to recall, causing great strife for the government? Even a rough location would be sufficient. We can offer that in the case of encryption. There are countless news articles about police not being able to decrypt data they deem important.

Without that, it doesn't matter. Laws are not created based on imagined situations that you can dream up. They only are created after something has become a problem. You can use a perfectly impenetrable vault all day long and as long as the government doesn't want in, it is never going to care.

Of course, the greater subject is really about houses, not vaults. The government has good reason to want to get into your house. For example, you might perish in it, and it needs to get in to deal with your mess. This is a relatively frequent task placed upon government to carry out. If you've made your house impenetrable, government isn't going to remain amused for long. If the government starts encountering that problem often, absolutely it would become illegal.

It is not illegal today because it has never posed a real problem.