Driving car on D3 in the highway bad for engine?

I have a 2000 Honda Accord ex. Recently I have a vacumm issue while driving in d3 but not d4( not so far ) but I have driven the car on the highway in d3 several times because one of my friends who works on cars told me that it saves gas if I had done that and i was online and read that it's better for my car if I have start with d3 then shift it to d4. Stupid and I know not to believe everything I've read online. Anyway those trips are 30-40 miles and I driven it on d3 on the highway about 8 times. About an hour long trip each time. Did I do major damage to my car and is the vacuum problem because of my driving?

Read more: Driving car on D3 in the highway bad for engine?