Bryce can be done in a long day trip. The best time to see Bryce is around sunset and sunrise when the colors come out. I would suggest this be an overnight.

It can be done as a day trip. You leave at 7am you get to Bryce by noon driving I-15 to UT 20 then US 89 to UT 12 (4 hrs + 1 hr time change), You are there for 4 or 5 hrs then leave around 5pm before sunset and try to get to I-15 in daylight going back the way you came and you will be in Vegas around 8pm without stopping but you could stop to eat dinner in Cedar City or St George for an hr.

Monument Valley is farther and not a good day trip. Similar to Bryce its best in late day sun and early morning sun to bring colors out. The drive there isn't 4 hrs but more like 6 hrs wheels turning + 1 hr time change. Given 6 hrs you figure you need to stop to eat and get gas.

If you leave at 7am you get there around 2:30pm local time, then if you leave at 6:30pm gets you back to Vegas at midnight.