Travel into and within Vietnam
Vietnam has two major international airports at Hanoi and Ho Chi Minh City, it takes little over 2 hours to fly between Hanoi and Ho Chi Minh City. If you choose to visit the whole of Vietnam, you can fly into one and fly out of another.
Domestic air connectivity is quite good. Trains are also quite popular with tourists, especially the trains with private carriages attached. I travelled in one of such overnight train from Hanoi to Sapa. It was comfortable journey, with choices of 2 or 4 sleeper beds in the compartment. There are luxury choices available on some of the popular routes. Best to reserve in advance.
For Sapa, one has to alight at Lao Cai station and take a Bus/taxi from there to Sapa town. There is no need to prebook. I preferred taxis as the rates are reasonable. You can also prebook private transfers though that will cost more. For the return journey, I booked a luxury car in Sapa for transfer to Hanoi as better trains only run during night.
For local/outstation commute, taxis or tourist taxis are convenient. Renting motor bikes or bicycles is also popular with tourists.
If you look at the tourist map of Vietnam, major attraction are clustered around Hanoi, Ho Chi Minh City and central Vietnam, almost midway between the 2 cities. For many western tourists, backpacking through Vietnam for weeks or even months is a popular option. I would think if you are visiting Vietnam, Ha Long Bay and Ninn Binh should definitely be on your list. You can also consider combining Angkor Wat in Cambodia with Vietnam.
When to travel
I travelled in the month of April. Hanoi and Halong Bay are a bit warm in April but manageable. No rainfall. Sapa was pleasant with occasional showers. February and March would also be good months to travel, in addition to the winter months, though Sapa can get cold in winter. Avoid May to September when it rains heavily and is hot and humid.
Accomodation
Hanoi
Plenty of options though you will not find many global hotel brands. There are many good boutique hotels but do check their rating and location. I chose to stay in a boutique hotel in Hoan Kiem(Old Quarter area), with the Hoan Kiem Lake within walking distance. This is a commercial area with several restaurants and a bustling market. Bit crowded yes.
Sapa
Small town, best to stay in the central area so you walk to the restaurants.
You can very decent rooms in a 4 star boutique hotels for VND 1.5 m or so per night. 5 Star hotels are likely to be above VND 2 m per night.
Ha Long bay cruise
I would recommend a 2 Nights, 3 days cruise. Choose a comfortable room, with a balcony or atleast big openable windows. Cruises rated 4 star(mid range) or 5 star(luxury) are better, and some of the luxury cruises even have a spa onboard. A 2N, 3D cruise will cost upwards of USD 200 per person for mid range while a 5 star one will be USD250 plus per person. Costs are higher during tourist season and on a prorate basis, 2N, 3D cruise cost is lower per night.
Choose a trip that involves a free/paid pick and drop from your hotel in Hanoi by the cruise company. It is 4 hours drive to Ha Long Bay. On the way, they stop at a shop selling beautiful artifacts. Though meant for tourists and bit overpriced, the quality is good. Boats are generally modest, with 20+ plus rooms, open deck but very few will have any other entertainment options. But then, you are there to admire the scenery, right?
Vegetarian food is generally available
Generally available and but don’t expect any variety in vegetables. I did try but could not take fancy to the bland Vietnamese vegetarian stuff; good once in way but not everyday. On the Ha Long Bay cruise also, vegetarian meals were served but I was fed up after 2 days. There are Indian restaurants in both Hanoi and Sapa but don’t expect great food.
On day trips outside Hanoi and Sapa, finding a proper vegetarian meal is not easy. It is not just about availability of vegetarian stuff, but also use of different vessels, oil etc. to cook vegetarian food.

Street food is popular in Vietnam, but I found the menu to be largely non-vegetarian. In any case, I am very fussy about eating street food on travel.
The prices are quite reasonable, with an average vegetarian meal for a couple costing ~ VND 300k or less.

Taxis/Car rentals
Convenient to move around but do agree the rate before getting in…many taxis refused to be engaged on meter.
For day trips, I generally preferred to engage a car through the hotel I stayed in. Sure, it may not always offer the best available rate but it is convenient and safer.
I was told it is not easy for foreigners to rent a car without a driver. In any case, given the on ground reality, it is best to hire a car with driver.
Currency-feel like a millionaire
This is one country where you will be carrying million bucks in your pocket which unfortunately, can only buy a decent meal for a couple in a good hotel. A million Vietnamese Dong fetches only INR 3k! Little over USD 40. Handling a million denominated note felt a bit nostalgic as my thoughts travelled back to the year 1999 in Italy where I first held a million Lira note in my wallet. Don’t recollect what it was worth at that point of time but was not much.