Expanding a bit on what others have said, I wonder if 5 days in Vegas might be too much. So much there is aimed at people 21+ that I think your teenagers might get frustrated with the limitations on what they can do and where they can go. I think Dave's idea of looping around to some of the Utah parks before the Grand Canyon might be a better use of a couple of those days.

I'd also agree that I think you'll want more time at the Grand Canyon, and I'd much rather have more time at the National Park ($25 for your entire family) than add in the West Rim/Skywalk ($300-400 for your family). If you were really pushed for time, then just spending a half day at the Canyon would be ok, but with the time you've got, I'd spend at least a full day there. Its easy to underestimate just how massive the Canyon is, and how many viewpoints there are to see. For that matter, on a trip with Teens, I'd give some serious consideration to hiking into the Canyon itself a little ways. You don't need any special tools or skills, other than some comfortable shoes and some water, and you don't even have to be in amazing shape to walk down a little ways. Even going a half mile to a mile down into the Canyon is a pretty amazing experience. Just remember, it is always more difficult going back and never try to go to the river and back in a day!