It wasn't quite shoulder to shoulder.
Vardy lent into Rafeal's back while running at a million miles an hour.
It game him an unfair advantage and, when you're running at the pace Vardy was, a push in the back would knock Wes Morgan over.
It's a foul, but the main problem that United fans should be looking into is the naivety of Rafael. He makes the referee give the penalty. He shouldn't have made that poor tackle. Regardless of what happened before it.
But like you say, the referee had absolutely no bearing on the result so it's a waste of time talking about this.