Really? A couple of gigs of space is super cheap. Cheaper, imho to upgrade your drive and clone it over than to delete/uninstall the older version....which you can do with no ill effects. Each version installs uniquely and can be uninstalled uniquely as well.
I always keep the last couple versions of Vegas... For super large projects, I actually do all of my time-consuming cuts & color work in an older version of Vegas that is rock stable. Then, I load a copy in a newer version of Vegas that has the FX, render codec, or faster render speed I want. One year, I couldn't even get a large project to load into the latest release of Vegas without an error message but I was able to finish it in the older version without any issues. Eventually they released an update that fixed the issue, but in the meantime I had deadlines to meet...
+11. They co-exist without issue, so I let sleeping dogs lie. Some plug-ins certainly can be compromised if installed with an earlier Vegas version (been there - done that).