Are you running Windows?
When you installed some of your VST's, they wrote to the Windows registry. Moving them now may corrupt those installations and you will likely find your self without those VST's unless you re-install.
It will be best that you uninstall, re-install, for best results. However, you're not going to save much space except for any that included sound libraries that can be installed to the external disk. .dll's and .fxb's, etc take up little room, so you will do a lot of work for basically no gain.