i've given my old version of 9 to a friends son - he's shows great promise - and since he lives in another state (not a drugged one thankfully) i like to install 9 back on my system so i can do walk throughs with him.
can i safely install an OLDER version AFTER having installed the latest?
Sure. I want to say that 11 was the last version to do a major install/setup change, so you might need to reinstall some plugins or point some Vegas folders to where the plugins are. But, I've (at one time, installed older versions with newer on there & nothing overlaps any more (I think it was 7, 8 or 9 where all plugins were in a shared folder vs separate folders, and I think 11 or 12 where some default folders for plugin searching was changed).
Only thing to worry about i can think of is that the older version will steal the .veg file type association so that clicking a .veg file will launch that version instead of 13. Pretty easy to change it back though.
The rumor i've seen posted (and mind you, it's just a rumor) is that it was too close a product to a some other software and may have been stepping on some intellectual property toes. Rather than face huge battles, it was quietly pulled instead.
There were also mentions that it would be replaced with something far better, but so far that hasn't come about.