To activate column mode in vim press Ctrl-v. Now mark you desired columns and execute commands on it.

      #
 ////###  ///
  ///####  //
  ///#####//
  ///####//       For example ctrl-v starts column mode
 #///###//##
##///##//####     j marks the column below 
 #///#//####
  /////####       and r replaces the columns with the following character
  ////####
  ///####
  // ###
      #

Further useful commands in visual mode:

I inserts text before selection
A appends text after selection
s replaces block with typed text

To append text at the end of line use ctrl-v followed by $.