I'll say this to the notion that the Dolphins need Olivier Vernon to be great in order to have a successful defense...need to do some historical research.
The Dolphins had a pretty good defense in 2008 that allowed 19.8 points per game and got them to the playoffs. The only pass rusher worth noting on the team was Joey Porter. Matt Roth certainly wasn't a guy that could put his hand down and rush reliably on passing downs. The only defensive lineman that could make any headway against the passer was Randy Starks. But Joey Porter was the guy.
Then in 2010, we again had a pretty good defense that allowed 20.8 points per game, and the only pass rusher was Cameron Wake. Koa Misi was a rookie that year and he was no better at rushing the passer than he is right now. Randy Starks was again our best pass rushing defensive lineman.
Now in 2011, we had a pretty good defense that allowed 19.6 points per game. Again, Cameron Wake the main pass rusher. I guess some would try and say yeah but we had Jason Taylor opposite him. But in reality, Taylor was no better from a pass rush standpoint in 2011 than Koa Misi was in 2010 or Matt Roth in 2008, the two guys that were functioning as pass rushers opposite Wake. Taylor rushed the passer 345 times and produced 7 sacks, 12 hits and 16 pressures. Misi in 2010 rushed 254 times and produced 5 sacks, 7 hits and 20 pressures. Roth rushed 205 times and produced 5 sacks, 4 hits and 18 pressures. Misi and Roth averaged a play on the quarterback every 7.8 rush snaps, where Taylor averaged one every 9.9 rush snaps.
In actuality, Miami does not NEED a second pass rusher in order to be a top 10 defense. They have enough on roster between Koa Misi, Jamaal Westerman and Olivier Vernon to accomplish what they need. I think if they want to be a top 3 defense they'll need Vernon to step up and be a great pass rusher in his own right...but they don't need that to be a top 5 to 10 defense.