If after installing your npm successfully, and you want to install VueJS then this is what you should do
after running the following command (as Admin)
npm install --global vue-cli
It will place the vue.cmd in the following directory
C:\Users\YourUserName\AppData\Roaming\npm
you will see this in your directory.
Now to use vue as a command in cmd. Open the cmd as admin and run the following command.
setx /M path "%path%;%appdata%\npm"
Now restart the cmd and run the vue again. It should work just fine, and then you can begin to develop with VueJS.
I hope this helps.