If there were vents in the first place, and they were sewn shut, yeah, you can probably have them reopened. If there were never any vents to begin with, though, probably not. They require a significant amount of overlap and there usually isn't enough seam allowance to construct it.
My approach with vintage items is that it's fine to alter them to make them more wearable, as long as the changes can be undone later. Slimming pants is fine if you make sure the tailor leaves the extra material on the inside of the seam and doesn't cut it away. Narrowing a lapel, OTOH, would be a crime.