If your intention is to improve your vocabulary ... then set that as your purpose knowing full well it will come to pass.
I too in the beginning, use to PR the dictionary every morning at work, my original intention was to work on my Photofocus and something after several months magically began to happen.
Part of my job involves communicating via email at work. What I was beginning to notice that I was using words and crafting sentences in such a way was not my way.
Something inside me had changed, my use of newer words to craft sentences and communicate in much clearer ways.
My suggestion to you would be to PR the dictionary every single day, as often as you can (use to take me less than 5 minutes everytime, several time throughtout the day) and you will begin to notice some wonderful things happening ...