My own choice for the most impressive World Heritage Sites in Vietnam is Halong Bay. I have been to Halong Bay twice and I really love this place.
The first time I went there was when I was in high school. The trip took about 2 days. At that time, we stayed on a boat cruising around the bay. That was such an amazing experience because from the boat, we could see lots of beautiful islands, especially the famous one named Ha Long Bay
The second time I visited Halong Bay was with my family. We booked a room at a hotel near the bay and arranged a cruise tour to visit some islands during the day and stayed overnight on a boat. That night we had a party on the boat with our friends and we ate lots of delicious seafood there. It was such a great time.
That's why I love Halong Bay so much because it has many beautiful islands of different shapes and it is also very convenient to go there by road as well as by train or airplane since it is quite near Hanoi city
In my opinion, if you are looking for any places to visit in Vietnam, you should definitely come to Halong Bay.