i know accomodations can be tricky in the busier parks and yosemite is a quite a drive in and out so not easy,but possibly staying at lee vining instead of mammoth lakes may give you a bit more time in the park. be sure to take the time to drive up to glacier point and take in the view,it's amazing.
another possibility for the grand canyon would be to leave vegas in the afternoon,1 day early if you could find accomodation in tusayan,which is right on the doorstep of the south rim. or once at the grand canyon a big part of the south rim experience is "desert view drive", heading east with some great viewpoints and ends at the old watchtower. it would be posible to leave by this exit which would create a loop by heading south to flagstaff on hwy 89.another possibility to maximize your time at the canyon would be to check out accomodation
here
and then head off towards sedona etc the following day,rather than having to leave the canyon early and miss this part. :)
sorry if i'm confusing matters,i just thought i would offer some options. lol!
have a great trip!!!!!!!!