Here is a page which is 64% full, no justification applied.
With Dorico’s default vertical justification settings (60% in the first box and 80% in the second box), Dorico adds space both between the two systems and between the staves in the systems.
If I lower the second value so that it’s less than 64%, Dorico only adds space between the two systems.
This is basically the same as the first image, except that the two systems have been spread apart by a lot of space. And I think this is what you want to avoid, right? The way to avoid that is to make sure that the second threshold in Vertical Justification is higher than the frame fullness, as it is for the second image.
Of course, different pages will have different fullnesses, and the vertical justification setting applies to all pages in the layout. If you don’t want Dorico ever to produce something like the third image, then set the second threshold to 100%.
Here’s a longer discussion about how the justification settings work, if you’re interested.