It will really depend on what airline you fly on. I have been flying for years and I am over 300 lbs (I have been between a size 24 and 30 each of the times that we have flown). In the past 5 years, I have been on about 30 flights - including international. Each airline has their own policies on when they require someone to have an additional seat. You will need to check on the airlines website. I have flown Delta, American, Continental, and US Airways without any issue - and I always need a seat belt extension, which I automatically ask for from a flight attendant as soon as I am on the plane, and no one has ever made me feel bad about asking for one either, they have always been very friendly. Southwest is really the only airline that I have heard of people being forced to buy a second seat and because of this, I refuse to fly with them.
The suggestions that I have for flying is that I have found that an aisle seat will give you a little bit more room since you can extend into the aisle for most of the flight and you have more head room since the overhead bins are not directly above you - you will just have to move your arm/leg when people are walking by or the flight attendants come by with a food/beverage cart. Also since you are flying with your hubby, as soon as you are seated, you can raise the arm rest between you too and then it seems more like a love seat then a movie theater seat.
You can go to websites like www.seatguru.com/
and check out the specifics on the plane that you are going to be on. The seats in each airline will vary in size, but I would associate most of them to the size of a movie theater seat. What I have done in the past is take a ruler to a chair and measure the width and compare that to the size listed for the airline seat.
As to the answer to your question, I think that you will be fine - I am actually a little bit shorter then you and have always weighed more then you on any of the flights I have taken in the past 10 or so years. Just take a deep breath and have a great time in Vegas!
"I have not failed. I've just found 10,000 ways that won't work." - Thomas A. Edison