Is there a way to set a default Vault for my searches

Whereas I suggested adding an Item with a specific username, password, and website to test, you mention only that you "did this with a password..." with no details of what you did or the password, or what happened when you tried my suggested test.
This makes addressing your reported result practically impossible.

Given the possibility that the password in question was a bad one in that it used private or personal information, then not revealing it is of course the right choice. In that case, it should not be too difficult to find another compromised password which you can share; that is unless they were all that bad.

In any case, please report the results of adding the Item suggested.

The blog begins,

While this data appears to be recycled from older breaches, ...

so it's not a matter of whether or not there was yet another discovery of a large collection of breach data, but whether there was discovery or publication of new (but not necessarily recent) breach data. In this case it appears that there was not, hence there was no new breach data added to HIBP , and thus nothing new for Watchtower to find and report.

If your credentials are in any of the breaches recorded by HIBP, then Watchtower should tell you.