SVP "steals" nothing unless you ordered it to do so by _manually_ running "set env. variables for vapoursynth" command, which shows a _huge_ warning message you must read _before_ continue.
If you didn't read it - blame yourself.
SVP also doesn't _install_ any Python version, there's only a portable Python sitting in its own folder.
> How do I fix this so all other software will also work?
Remove SVP 4\mpv64 folder from PATH and PYTHONPATH variables.
> If I am to install the full version, and disable yours, what do I need to add
Vapoursynth