Can vegetable oil be used in place of butter?

I just happen to have one of those Betty Crocker cookie mixes on hand today, and the ingredients list at the back calls for 1/2 a stick of softened butter. I don't have any butter to spare today, but I do have a bit of vegetable oil, and I know that the two can be substituted sometimes without incident.

Can I do so here, or should I do as the package recommends and use butter?