Fairfax County’s vision for Tysons is set. Developers are lining up to put that vision into action with mixed-use, transit-oriented projects. Now, it needs someone to bring it all together, to make Tysons a place. That’s where David Diaz, a former U.S. Army intelligence officer in Iraq, comes in. Diaz, 48, took over in March as the Tysons Partnership’s president and executive director, replacing the retiring Michael Caplin. He arrives from Raleigh, North Carolina, where he served as head…