Open an existing file from Vim

vim [your file]

If this isn't working for you, make sure you have it installed with:

sudo apt-get install vim

If you're already IN vim do

:edit [your file]

Additionally, to move back to the previous file after you finish editing the new file you can use

:e#

This will return you to the previous file

You can also use tabs and split views:

:tabe /path/to/file

Effect:

:vs /path/to/file

(or :sp for a horizontal split)
Effect:

Use :tabn, :tabp to navigate between tabs, and CtrlW with the arrow keys to navigate between splits.