I was camping there for a few days last year in July, and you shouldn't be disappointed, assuming you are into long and strenuous hikes, because that's really the only way to really enjoy this park. First of all entering the park from the east entrance is amazing as you go through tunnels and never ending canyons. Since you're coming from Vegas, going around the park just to enter from the east might not be very practical (it's probably quite long), but I'm just telling you. As far as staying in Zion, you can do pretty much anything from camping to staying in hotels at Springdale, which is a small town basically right at the west entrance of the park (The park shuttles actually go as far as that town).
Now the hikes... This really depends on how fit you are, how much effort you want to put into it and how comfortable you are with heights and narrow passages with long falls below (not that there's anything really dangerous). I would say that if I had limited time and had to choose what do to again, I would definitely do the Angels Landing trail (Great hike, end up on a small island in the sky basically surrounded by canyons). And you really shouldn't miss The Narrows, where you can walk for hours in a river surrounded by steep cliffs. This is the main attraction of the park, so it is really crowded at the beginning, but if you go far enough, you filter out the lazy people. I think we went 2-3 hours into the river without any equipment (the water usually stayed at knee level) before heading back, it is very refreshing, and I would save it for last.
If you want to really go hardcore, you can try Observation Point, this is probably the longest and steepest hike. Bring A LOT of water or you'll collapse (we had to give a guy some of our water for the way back, he was literally not gonna make it in the 40C heat).
The shuttle service is really a good thing, apparently the traffic problems were awful before they switched to shuttles. You can catch them at several locations in the park every 5min or so, there's essentially no waiting.
Anyways, It's an amazing park (I liked it better than Grand Canyon, which is more impressive, but not as beautiful in terms of colors and rock formations). If you intend to stay more or less at ground level and do shorter hikes, I'm sure there's a lot to see, but I have limited knowledge about those. Seeing the park from above (Angels Landing, Observation Point) really shouldn't be missed, but it takes on the order of 4 hours round trip.
Hope you enjoy it