Vontae will never have the dedication or mindset to live up to his considerable talent. He'll have his good days, like the game against Denver, but the next contract he signs is going to be a disappointment for the Colts or some other club.
As for Marshall, that trade really sucked for the Dolphins, but I'm not sure there was a choice. He would have been unhappy staying here and likely would undermine the confidence of Tannehill or any other young QB. Just like Oakland and Randy Moss...the Raiders knew they were getting fleeced, but what was the alternative?
The problem with the Dolphins is organizational, top to bottom. We do not have a coherent and supportive power structure to handle these issues when they come up. People are out to save their own skin and will throw others under the bus to do so.