Here are some comments based on an assumption from your talk about off-piste at Chamonix that you like very challenging terrain. I also have first hand knowledge about January in the Eastern US.
From New York I'm guessing you'd like to do about 3 days of skiing: there are lots of good options, but maybe try Sugarbush, VT ski area, about 325 miles (6 hours drive time). In the Eastern US good off-piste conditions are often problematic due to lack of natural snow. Jan 7-21, 2007 is not bad, but a little early for best snow. Sugarbush has a nice quantity of challenging to very challenging on-piste terrain (covered by snowmaking), but also puts you very near Mad River Glen ski area (renowned for challenging off-piste) and if Mother Nature cooperates you can try a day there. Jay Peak ski area is also a good bet for challenging off-piste, but it is somewhat isolated (about 2 hrs beyond Sugarbush) and would not offer as much on-piste terrain in case natural snow is sparse.
From Las Vegas I'm guessing you'd like to do about 5 days of skiing: You have two good options in totally different directions.
1) drive to Mammoth Mtn, CA ski area, about 300 miles (7.5 hours), spend all 5 days there. It is a big place for the US (also has smaller nearby companion ski area June Mtn) with good snow and challenging terrain. If you must have variety, drive about 3 hours further north for a day at Kirkwood, CA ski area, also good snow/terrain.
2) drive to the Salt Lake City, UT area (skip Brian Head or visit it coming or going to SLC, it's highly regarded, but small). SLC is about 400 miles, but 6 flat/fast hours from Las Vegas. Here you can ski a different nearby ski area each day including Snowbird, Alta, Solitude, Brighton. Usually great snow and some very challenging on/off-piste terrain.
Note that Jan 13, 14, and 15 is Martin Luther King holiday weekend and could be a little crowded, especially at Eastern US ski areas, otherwise your mid-January dates are typically crowd-free throughout the US. Personally, I wouldn't hire a 4x4 vehicle. Most driving you'll be doing from NYC or Las Vegas will be on flat Interstate highways where a smaller car will be cost effective. Only the last few miles are in the mountains with a moderate chance you will see difficult driving around Mammoth or Salt Lake City, but there are options such as local buses in case of surprise heavy snow. But if you want to play it safe and have 2 or 3 travel companions a 4x4 SUV is a sensible choice. If you have the flexibility, decide your actual choices only a day or two in advance based on local snow conditions at the time.