Yes I agree! The world is an amazing place and there are so many places to explore.
My Thailand experience was limited to Bangkok so it is hard for me to provide you with a fair comparison however I think your friend's recommendation to see Vietnam before it becomes more popular is good. However don't be misled there are plenty of tourists in Vietnam already. In fact, thousands of people a day visit the World Heritage site Ha Long Bay, an amazingly beautiful natural wonder. If you are visiting Vietnam on a tour I am certain you will visit this site however if you are organizing your trip on your own, then make sure to include it on your itinerary! Actually, it was here that I decided that I would go to Antarctica to see the last untouched frontier of our world and since Antarctica my life has been a whirlwind adventure.
How many days will you stay in Vietnam? Are you going via cruise, tour or solo?