Can I use butter instead of vegetable oil when baking brownies from a mix?

That is a great question.  You can absolutely substitute butter for the vegetable oil.  Use the same quantity specified in the directions (for example, if it calls for 1/3 cup of oil, use 5 1/3 tablespoons of butter).  Melt it down, then let it cool a bit.  You might not ever go back to oil!