So, what was the result here?
I've found in my vsdelta folder some VPJ files, but not most.
I found if I have open the VPJ as a text file and save it a couple of times to create the p.VPJ.VSDELTA, then when I do Project->Properties and change things, Slick does keep history - as long as I keep the VPJ file open as a text file as well (versions are "Created by Auto Reload").