sounds like a tall task. I know some VB and I don't even know where to start. You could save yourself the trouble and go here:
http://www.whatifsports.com/nfl/default.asp
It also depends on how you would rank teams. If you have passing, rushing, special teams coverage, special teams return, defensive rushing, defense passing, coaching as rankings. Then you would assign values to each other and see how they matched up. You would have to assign maybe a point value to the differences, have a random point value be determined by the difference.
example: 80 Pass offense vs. 60 pass def = 20 pt difference. Then you would have to assign a point scale for a 20pt difference, say 30-60 since 20 pts is a large difference. You would randomly pick a number between 30-60 and then that would give you a point value for the qt, half, game depending on how you want to do it. I would also maybe give different weights to the passing, rushing, special teams values. Maybe 60/30/10 since its easier to score through passing. Then assign a point scale for TD's, FG's, safeties. Maybe every 100 points gets a team a TD. Maybe when the difference between ratings is in the favor of the defense and they randomly get the maximum number of difference points you award them a safety. The complexity of this program is immense and you could get swallowed in it. Especially if you wanted to have the user pick a run or pass, then run the ratings against each other and assign a yardage total based on qualifiers.