The 1x2" framing is not that hard to put in and it really is the correct way to do it.  Spend some time reading posts here on how and why its done and you'll see that it needs to be done this way. 

Gard is the local expert on this stuff (he wont say that but the rest of us will), search his posts and your can learn a lot. 

You only get one shot at it so do it right the first time. 

Good luck.

I just don't see the addition of special vapor barrier as being a really critical part of finishing the trailer.

I've research enough trying to argue with a local building inspector that I think I can give some idea of what happens. My local city inspector requires me to install a visqueen vapor barrier on all external walls of the homes I build.  He is the only city in the three county area that I build that requires it. From a builders stand point I've argured a loosing battle with him. Based upon information I've gotten from the insulation companies and other trade organization an additional plastic moisture barrier is not required and doesn't add any value . Any wall that I layer the visqueen over has, to have the sheetrock screwed on. Vs all the walls with no plastic where we glue and screw the sheetrock. The glue is more effective and stops the nails from popping. So I always have more nail pops on exterior walls in this city than that I do in any other city because I can't glue sheetrock to visqueen. 

A vapor barrier is designed to keep moisture from pushing from a High moisture area into a low moisture area. In a home we want to keep moisture from our shower, steam from cooking, even moisture exhaled in our breath from pushing into the wall cavities. So hence the concept of installing a sheet of plastic over the walls before we add sheetrock.  

Studies have shown that more moisture enters the wall cavity from one single gang electrical outlet, that enters the wall through an entire sheet of sheetrock that has been coated with an acrylic latex paint.  If you have one switch or outlet in a wall, That penetration defeats the purpose of the visqueen.

With the large number of penetrations in a horse trailer living quarters, ( door, windows, electrical outlets etc) You will not save much in moisture movement by installing visqueen.

The main purpose that we are looking for in a horse trailer is a thermal break so that we don't get condensation on the walls. This moisture is entering the trailer as humidity from outside air, that enters through doors and window, and from cooking and our breathing. It condenses on the walls when warm air touches a cold wall. You will have more comfortable and dryer trailer if you spend the time and money improving the thermal break in the wall cavity than you ever will installing a moisture barrier.

Trailers are built with steel or aluminum struts that the outside skin is attached. These struts transmit the temperature difference directly to the inside wall material, where condensation occurs. Try to sandwich some kind of thermal break over the metal studs and under the final surface of the interior walls to prevent that transmission of cold from the outside to the inside wall.