Hi,
3 weeks for travelling around Vietnam will enough. Personally, you should travel by train to HCMC to save money. Furthermore, to avoid terriable things, you should book or make itinerary before travelling. Around Vietnam, you can visit Hanoi-Halong-HCMC-Mekong Delta (Cu Chi tunnel-Cai Be/Phong Dien floating market- Ben Tre)- HCMC. I think should think about homestay which save more money for you, also know more about culture and real life of local person. Homestay is trending of travel Vietnam.

OP, another thing to be weary of if you want to avoid stomach issues are condiments. Those sauces on the table probably aren't very hygienic and/or fresh especially at a hole-in-the-wall, and likely cut with tap water. When in doubt, I would err on the side of caution.

Another local nuance... if you like your beer cold, ask them if they have any cold ones or to put it on ice. Otherwise, they give you a glass with an ice cube in it, for you to pour your warm beer over.

I would also arrange tours while you're there. You can negotiate, and also get info from other travelers who have taken tours on which ones they recommend... and which to avoid. Your hotel can also help with tour arrangements.

That's frankly one of the worst pieces of advice I've ever heard. The money saved is miniscule compared to the time lost and discomfort suffered. Unless you have an absolute need to travel by land for 30-35 hours (if it runs on schedule), flying is the way to go.

Save money by spent more time is not a smart way to live, in Vietnam price are not to much for foreigner, except you scare of trick by local people. Spent a little bit more and you can enjoy your trip comfortable.

I stayed at Hotel Metropole in Hanoi. Lovely historical hotel but the prices were definitely on par with the west, but it was stunning.

We did a tour with Awesome Vietnam Travel called "Food on Foot". It was absolutely fabulous for the $AUD25 approx it was. It was 3 to 4 hours of walking around Hanoi eating at various places. It was a great start to show us where to go and what the prices were. It gave us confidence to go out on our own to eat. Some of the fabulous stir frys at local establishments were AUD$4. I have a delicate stomach and I didn't get sick. My husband ate duck eggs, pho, egg coffee and an assortment of street food and never got sick. We might just of been lucky. We loved this little tour company and then used them for other tours we did such as Chu Chi tunnels.

We did a walking tour in Hanoi starting at about 5 am and it was very interesting to see the city waking up. A highlight was at about 6 am at Hoam Kiem lake where everyone was exercising, dancing, marching up and down on the spot and a favourite of mine was a big line of old ladies slapping each other on the back, turning to face the other way and repeating!

It is easy to get a Visa. In your own country you can get a official letter of approval to get in to the country then when you land get your Visa. I found it easier to get the full Visa whilst in Australia, although it did cost an extra $30 to $50, but it saved me having to line up with tons of people from the plane.

I loved Vietnam, particularly Hanoi. I was there for just over 2 weeks. About 9 days in Hanoi and the rest in HCMC. I arrived in Hani and flew to HCMC. I guess what you choose has to depend on the time it takes by train or the cost of flying. I was pleasantly surprised with Vietnam Airlines.

I loved Hanoi as well.

Only two days there, but it was amazing. Friday night lights and motorbikes around the lake, wandering the old quarter, and 7AM Sunday mass at the Cathedral.

I stayed at a small hotel--Josephs--near the Cathedral. It was $50 CAD per night, as I recall, and it was perfect---spotlessly clean, good AC, comfy bed, good shower, pho for breakfast etc etc etc.