I've read that when I buy a cast iron skillet I need to "season" it with any vegetable oil, or buy from a company that has already done this process, i.e cooking the vegetable oil at really high temperatures. Doesn't this produce trans fats? Will this leave trans fats in my food afterwards?
Trans fats, as in hydrogenated fats, are not created by heating oil. The process of seasoning a pan involves polymerizing, drying, and carbonizing the oil, which is different from hydrogenation of the oil (how trans fats are created).
Per @craPkit, there are confusing references to trans fats and overheating oil. However, overheated oil is oxygenated, not hydrogenated. These oxygenated oils are confusingly referred to as trans fats in some literature for technical reasons, but they are different from what is meant by "trans fats" in health literature (that is, partially hydrogenated fats). That said, oxygenated oils are probably bad for you, just in a different way (primarily for your cholesterol level).
Additionally, if you have seasoned your pan properly, oil shouldn't be coming off into your food in any significant quantity. The whole point of seasoning is to create a hard non-stick surface on the cast iron that doesn't come off; if it is coming off, you did it wrong.