Two major comments:

1) Why the SRU theme?  You could have just as easily done this story in the Medallion of Zulo universe, and it would have made more sense, aside from the ending, which could have been subtley re-worked and made to fit.  Actually, come to think of it, the whole intro with the girlfriend wasn't all that necessary, because it was never followed-up on.  By using it, you just cluttered the story up more.

2) Nickie was introduced too early.  You didn't mention the brother until his jersey was required for a story element, and you shouldn't have mentioned the sister until her clothes were introduced as a story element.  

The rest is mainly taste more than anything... I would have liked the story better if Don had attempted something on either Shirley or Vinnie which "backfired," as your title had implied.  Perhaps he moves to give Shirley the medallion, and instead slips so that it's touching his hand and Shirley's blouse at the same time?  Perhaps he turns Vinnie into a girl to remove competition?  But like I said, that's just me.  If you followed this advice, you wouldn't have to remove the intro section, but if you want to keep the main plot of the story as it is, I'd lose it.