Which raises the question why the return trip was significantly longer.
Because they didn't return from Vulcan to Earth. If you recall the Enterprise was going to meet up with the rest of the fleet in the Laurentian system. When Kirk was dumped off & met up with Spock that probably took about 10/15 minutes in their first meeting, and then they had to walk 14 KM (approximately 8 Miles) to the nearest starbase which may have taken a few hours. So by the time they got through with Scotty and beamed back it seems that several hours had already passed and the Enterprise was still in route to its destination. So it was several hours off, thus their return trip would probably take several hours.