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.

The taste part of your question to me is most easily answered with another question: Does the vegetable oil taste like butter? If not, and I am fairly confident you will answer no to that, then yes, it will alter the taste. How much depends on how much you use. but most cookies use quite a bit of fat so the change would typically be noticeable.

There are things like butter flavored Crisco that are used as direct substitutes, but I suspect that is not what you are looking for in an answer. In most applications though, if you are going from a semi-solid like butter to a liquid vegetable oil you will likely have very noticeable changes in texture and cooking results due to increased moisture. And in many cases you also may lose chemical and browning effects. Some applications may work, but without experimenting, I would not expect to go from butter to a liquid oil entirely successfully.

You technically could, but I doubt they'll come out the way you intend. You'll achieve zero creaming aeration. Without seeing a recipe, I'd say you'll have greasy, crisp, dense, fried "cookies". Use coconut oil as a replacement if you have on hand, as its firm at room temperature, like butter.

Butter is an emulsion of about 75-80% fat in water with some dissolved proteins. If you want to replace it with oil, make a mix of 75% oil and 25% water, and whip it into an emulsion before adding. Might add some soy lecithin or other emulsifier to make it easier. Also might want to choose an oil on the more saturated side like coconut or palm.