http://stackoverflow.com/questions/1676632/whats-a-quick-way-to-comment-uncomment-lines-in-vim

o comment out blocks in vim:

• press Esc (to leave editing or other mode)
• hit ctrl+v (visual block mode)
• use the up/down arrow keys to select lines you want (it won’t highlight everything – it’s OK!)
• Shift+i (capital I)
• insert the text you want, i.e. ‘% ‘
• press Esc

Give it a second to work.

To uncomment blocks in vim:

• press Esc (to leave editing or other mode)
• hit ctrl+v (visual block mode)
• use the up/down 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 + left/right 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
• press Esc

Give it a second to work.