Thanks for your prompt response. No - I simply update and use the latest updated version.
My concern is that most often in other software (eg Adobe) an update updates the original installation and renames it, so by uninstalling the older versions I might 'disembowel' that which I have just updated.
If uninstalling the older versions isn't going to affect the v18 that I've just updated from v17, then I'll happily remove the older unused versions.

Ah ha yes - Of course - Plugins and them being possibly incompatible .... which means each installation is a separate standalone installation. Why didn't I think of that?

Oh and thanks re the info for 2 machines ... I wasn't aware. That situation is ME so I'm now much happier.

I had 15-18 sitting here and realized the other day I never plan to use 16 or 17 again so uninstalled them. I kept 15 as it has a few Hitfilm plugins that are locked to that version. Vegas installs each version of Vegas to a different folder so they remain separate and fully functional. Many OFx and VSTs are also stored in shared folders that work across versions (my Neat Video seems to be in its own folder for example.) Some need to be reinstalled after you install a new version of Vegas- double-check you aren't missing anything with your new version.

But see what's where for yourself: C:\Program Files\VEGAS

If you use the bundled effects packages that came with older version you will want to keep the older version as bundled packages may not work with upgraded version. I've been scammed into using bundled effects and then opening projects in newer version and finding out that the effect packages is basically a bait and switch scam.

I never purchase upgrades based on bundled effects ONLY on built in features.

Thanks All you blokes for helping and doing so promptly.
It turns out that my situation was same as ByronK - I should have realised BEFORE asking my question that each installation is separate. I shuld also have understood why. NOW I DO, so hopefully no more dumb questions!
THANKS CHAPS

Different programs are different so it's fine to ask how Vegas works. Happy editing!

I have an issue in that I have been running V18 for quite some time and yet on some occassions when i go to open an established edit file in V18 it shows up in my file explorer as being in V14! Sometimes it creates an error and won't load. The thing is the new project was started in V18 and all the video clips for the project were captured in V18 so why is V14 rearing its ugly head? How to fix?

@tom-edinger, that's a Windows "file association" issue. When you start 18, it should ask if you want to make it the "default". Say yes. Or you can manually change it in Windows.