You must make the trip to Halong Bay, it doesn't matter if you are on your own. Join one of the many cruise boats which do the business in this most beautiful of place.
We did a 4 night I think and joined about 12 people from around the world and had the most wonderful time.
It's not high class cruising at all just very friendly informal days spent chatting and having fun with like minded people.
We made friends with two ladies from China and met up with them in Hong Kong when we arrived there.
Vietnam is a very easy country to travel in everything is well set up for the tourist.
Have a great time but keep safe.