Can i use vegetable oil instead of butter in cookies?
You can, but there will be several differences as a result:


Taste is the most obvious. Depending on the oil you choose, you will either just lack the buttery aspects of the flavor or replace them with other flavors (nutty for peanut oil, coconut for coconut oil, etc.)
Texture and structure is the next most obvious. Butter, which is semisolid at typical room temperature, will provide more textural support for your cookies than an oil which is liquid at typical room temperature. You won't be able to "cream" most oils the way you do with butter to provide a lighter texture. Using a vegetable shortening like Crisco, or another oil like coconut oil that remains solid at higher temperatures than most oils, could partially address this aspect.
Moisture content is related to texture but worth being called out on its own. Vegetable oils are 100% fat; butter is about 80% fat, with most of the remainder being water (and a small amount of milk solids, about 1-2%). To best approximate the butter, you should use 80% of the suggested amount as oil, and 20% as water.


So if you have a favorite cookie recipe that you must make with oil rather than butter (such as if you're cooking for vegans, or people with allergies), the best approach would probably be to use 80% coconut oil (which for most recipes "75% and round up" would be close enough) and 20% water. This would provide a nice flavor (although one very different from butter), a melting point more similar to butter than most oils, and gets the moisture content right.