That should work. If you arrive in Vegas at 11:00, you'll be on your way in your rental car by 12:30. Depending on time spent in DV, stopping to eat, supplies, etc., you should be at the Portal well before dark in August. Plenty of time to set up camp. Two nights acclimating at elevation should be sufficient for a backpack up the mountain.
Most everyone spends the overnight at Trail Camp at 12K', but by August it's a ghetto of trash, noisy people, and used Wag bags. You might consider camping above Consultation Lake, just below TC. You'll see Consultation Lake on your left after you pass Trailside Meadow, then it will be hidden behind a rock berm. Peek over the berm when you're on the "staircase" and you'll find excellent campsites on the ledges just below. Quiet, private, and shielded from much of the wind that often hits TC, with a gorgeous view of the lake. Getting water is a little further trek than staying at TC, but it's worth it to me.
Be sure to have one of Doug's burgers with double-fried fries while at the Portal, and a pancake as well. They're both an experience you won't find anywhere else. FWIW, a part of a pancake broken up and stored in baggies makes great trail munchy.
