What is the difference between regular (vege. oil) and butter recipe cake mixes?
Butter gives more flavor, but oil provides longevity. 

Butter is about 20% water. That water will evaporate during baking and continue to do so even after they are out of the oven causing your cakes to stale quicker. Also the the fat in butter remains solid at room temperature. Once your cake has cooled the fat, now dispersed in your cake solidifies once again and makes for a bit firmer of a crumb.

Vegetable oil, however, is pure fat containing no water and remains liquid at room temperature. The moisture provided by oil will not evaporate during cooking and even after the product is cooled to room temp, the liquid fat remain and provide a moist crumb. Since there is no water to evaporate this product will last much longer on the shelf than one made with butter.