Hi Nicola
I am not going to Vietnam in April but have been twice before and I must say it is a facinating place. If you go on the boat trip around Halong Bay, it is well worth it because you can swim in the sea before lunch is served on the boat.
When you speak to the local people, who are very friendly, please do not be offended if they ask you questions like: "how old are you, are you married or have you any children". It is only because they are interested to learn and improve their English. I was once asked by a young lady, "do you have a husband?" The young people respect their elders.
I hope you have a wonderful and enjoyable holiday to Vietnam
Regards
Lesley