You are correct: V-Justify is set up to fully justify the frame. If it isn’t able to do this, it will return the spacing to the defaults.
The way V-Justify is written, however, happens to make it very easy to add the feature you request, which I’ve now done.
I’ve also fixed the issue with the inset in the text frame causing V-Justify to fail.
So please download the latest version (1.6.2) from the My Account page.
In this new version, I’ve added another modifier key: Clicking on the Justify button while holding down Ctrl (Cmd on a Mac) tells V-Justify to add the maximum space (or reduce the space to minimum, depending on the user-selected justify mode) and keep those changes even if the frame cannot be fully justified. I think this is what you were asking for.
Let me know how it goes!