Deciding whether to use vegetable oil or olive oil to cook a particular food is often quite a challenge. Vegetable oil refers to oil made from various plants, like canola or corn oil. Olive oil is made from pressing olives until they release their oil. The cooking differences between the two aren't immense, but they are worth noting.
The Smoke Point
The smoke point refers to the temperature that the oil starts to smoke. Vegetable oil has a higher smoke point than olive oil, which means for high heat searing of foods or deep-frying, vegetable oil enables you to cook the food thoroughly without worrying about the oil smoking and creating a fire hazard. Olive oil works better for moderate-heat cooking where the temperature won't rise too high. The smoke point for olive oil sits around 370 F, while corn oil is between 400 and 450 F, and canola oil is between 425 and 475 F.
Taste is another difference between using olive oil and vegetable oil for cooking. Vegetable oil is a neutral oil and won't add any taste at all to whatever you are cooking. Olive oil, on the other hand, has a distinct flavor and many cooks use it specifically to add flavor and a little fragrance to a dish. The taste of olive oil doesn't match perfectly with every cuisine, but that is ultimately for the cook to decide. For raw dishes and salad dressings, olive oil is usually used ahead of vegetable oil because of its taste-enhancing qualities.
The price is another difference between cooking with a vegetable oil and olive oil. Olive oil is generally more expensive, with some imported bottles of extra-virgin olive oil comparable in price to bottles of wine. Extra-virgin olive oil comes from the very first pressing of the olives and it will always cost more than virgin olive oil, or bottles simply labelled "olive oil" that are from later pressings. You can find reasonably-priced, quality olive oil, but most of the time it will have a higher price tag than the same amount of vegetable oil.
Use Your Judgement
In the end, it comes down to what you want in your finished dish. Where cooking is concerned, vegetable oil will meet your needs more often than not. You can heat it to a high temperature, it doesn't cost much and it won't alter the taste of your dish. For things like salad dressings or cold dishes where no heat is applied, your best olive oil is usually the way to go.