VI shortcuts

vi_commands.txt

To comment out blocks in vim:

press Esc (to leave editing or other mode)
hit ctrl+v (visual block mode)
use the ↑/↓ arrow keys to select lines you want (it won't highlight everything - it's OK!)
Shift+i (capital I)
insert the text you want, e.g. %
press EscEsc

To uncomment blocks in vim:

press Esc (to leave editing or other mode)
hit ctrl+v (visual block mode)
use the ↑/↓ arrow keys to select the lines to uncomment.
If you want to select multiple characters, use one or combine these methods:

use the left/right arrow keys to select more text
to select chunks of text use shift + ←/→ arrow key
you can repeatedly push the delete keys below, like a regular delete button
press d or x to delete characters, repeatedly if necessary

Undo:

u

Delete character under cursor:

x

V       - selects entire lines 
v       - selects range of text