I've found that typing in column mode to be very useful when wanting to comment out large blocks of text for example, in and apache virtual host config. Vi also has a column edit mode, that work very well for this type of scenario.
To use it, press:
Ctrl + V to go into column mode
Select the columns and rows where you want to enter your text
Shift + i to go into insert mode in column mode
Type in the text you want to enter. Dont be discouraged by the fact that only the first row is changed.
Esc to apply your changes
You will now see your changed applied.