In VIM, you can you use the key sequence YpVr= to quickly generate an underline.  For example, if you have a line in Vim:

My List of Things

You can change it to the following:

My List of Things

by using the following commands:

 Y - Yank current line to buffer
 p - Insert current buffer below current line
 V - Visually select the entire line
 r - Replace everything select with the following character
 = - Replacing character