Although there's obviously a good case for Macdonald, there's also a very strong case for Vrabel.
In one year, Vrabel took a team that was 4-13 and led them to a 14-3 and one win away from the Lombardi. Macdonald, on the other hand, took a team that in the year prior to his arrival was 9-8, improved to 10-7 but missed the playoffs, then had a slam dunk Lombardi season this year.
Being that the measurement is for one season, in particular the difference in the regular season W/L records, 4-13 to 14-3 is more impressive than 10-7 to 14-3, while not necessarily agreeing, I can understand voters choosing Vrabel.