Sequoia woukd be a good Sierra Nevada mountain park to go to from Las Vegas as an alternative to Yosemite, since it is farther south. There is no eastern road entry into Sequoia; from Las Vegas, you have to drive to the Bakersfield area (either the city or a very steep and twisty bypass that I wouldn't recommend, especially in winter). And the eastern road into Yosemite will still be in its winter closure in March, so you have to go beyond Sequoia..
Death Valley is only a couple of hours from Las Vegas, and you could spend at least a couple of days there because it is huge and has so much to see. In between is Red Rock Canyon NV, just west of Las Vegas (not to be mistaken for Red Rock Canyon CA which is just southwest of Death Valley. Near Las Vegas, other outdoor areas to explore are Valley of Fire State Park, Lake Mead and Hoover Dam.
Grand Canyon is 4-5 hours from Las Vegas and is also worth a couple of days. Or you could go to one or more of the southern Utah parks; Zion would be the closest and it's about 3 hours from town..
With limited time, I''d say you would have to choose either Zion or Grand Canyon (South Rim only; North will probably be in winter closure) because they are on opposite sides of the Colorado River and the crossings are hundreds of miles apart.
If you are comig in mid to late March, it will be wildflower season in parts of Death Valley (how good the wildflower season we can expect will depend on weather conditions over the preceding few months). I am not familiar enough with Zion or Grand Canyon to know when the wildflowers start to show up, but their elevations are higher so spring will arrive later. Sequoia will probably still have snow.
Not sure if you could really do justice to Sequoia AND Death Valley AND Grand Canyon (or Zion) in a week because of the long distances in different directions and the potential for late winter or early spring unpredictable weather. At least from my POV--if I go somewhere new, especially if I've driven a long way, I like to settle in for a couple or more days, get to know the place and see as much as I can. Let's see what various people here think.