By the way.. We now selected Grand Canyon, Bryce, and Momument valley to visit after Las Vegas. So no Zion.. is Zion a must see? If I have to pick three due to time constraints, what national park should I skip?

Monument Valley isn't a national park it is on Navajo land. I think Zion is nicer, but they are very different and both are beautiful. Zion is only 3 hours or less from Vegas, Monument adds hours of driving to your trip.

Monument Valley is a lot farther. Also, there are only two options for lodging there - the View, which is at least $200 per night, and Goulding's - if you can find an available room. I'm also visiting Monument Valley in September, and even though I booked my reservation the beginning of April, I had to rearrange my trip so I'd arrive on a day when there was an available room. There are other options further away, but they would require a bit of a commute(another 30-45 minute drive to and from Monument Valley).

So between time and money issues, you'd be best off skipping Monument Valley.

Geert-
Not sure you saw this:
We just came back from our babymoon 2 weeks ago... and were gone 16 days...
We have some that are on your list, here are mine..
Hotels:
Napa Valley: Gaia Napa Valley Hotel ($100)
San Francisco: Columbus Motor Inn ($100 with free parking)
Yosemite Valley: The Homestead (more than $200 a night but hey have less expensive rooms, it was awesome, about 30 min frm Mariposa Grove and 50 min from Yosemite Village)
Las Vegas: Bellagio (more than $200 a night, SO worth it!, don't go weekend, but make sure you're there a weekday... Venetian is nice also... love the location of Bellagio, even Paris, in the middle of the strip... We stayed at Mandalay before, amazing pool...
Sedona: Sedona Real Inn & Suites ($175)
Grand Canyon: Tusayan Holiday Inn Express Suite ($150) you can get a regular room also. about 10 min from the park...
Bryce Canyon: Best Western Ruby Inn ($125 internet special)
Moab: Best Western Greenwill Inn near Canyonlands ($120)

Plus or minus on the room rates.. check online also for specials..

We enjoyed all of them and had no complaints! Best 2 places were Bellagio and The Homestead... you can check my trip report if you would like more info.... We did Sacramento-Napa-San Francisco-Yosemite-Las Vegas-Sedona-Grand Canyon- Bryce-Moab-Salt Lake City-Rapid City... We did our trip in 16 days... but not as many places as you... http://www.fodors.com/community/unit...yce-canyon.cfm

http://www.fodors.com/community/unit...ty-summary.cfm

Avis is great for one way rentals....

I would skip Monument Valley, we skipped Zion and went off to Bryce, but I would do Zion over Monument Valley...

Keep in mind traffic in LA is horrible. We wanted to stop by, but ended up heading on to Vegas from Yosemite. Google Maps will give you time WITH traffic.. and it was an extra 4 hours +

I know someone who from Vegas drove to Zion, then stayed at Bryce... but you have the Grand Canyon in there, make sure the North Rim is open, it wasn't a few weeks ago, so we took the long route to the Canyon...

Thanks everybody for the great tips!

I would definitely pick Zion over Monument Valley! It's really spectacular, one of my favorite places. It also fits much better into the rest of your itinerary, as it's between Las Vegas and Bryce Canyon. Adding in Monument Valley would add many extra hours of driving, time much better spent exploring Zion and Bryce Canyons.