Twenty-one days on the ground is the length at which Vietnam stops being a choice between regions and becomes a question of what to add. The fourteen-day shape already covers the country north to south; three weeks buys you the places that get cut from it, plus slack for weather.
The fourteen-day spine
Start from the standard two-week route, because three weeks is that plus additions rather than a different plan. Hanoi; a northern excursion; the centre around Hue and Hoi An; Ho Chi Minh City. Open-jaw into Hanoi and out of Ho Chi Minh City, with two internal flights and the Hue to Da Nang leg by train.
What three weeks changes is that you no longer have to choose between the northern options, and the south becomes a region rather than a two-day stop.
Three weeks: the spine plus both northern excursions: Hanoi → Sapa → Ninh Binh → Ha Long & Lan Ha Bay → Hue → Hoi An → Nha Trang → Ho Chi Minh City → Can Tho & the Mekong Delta. (Enable JavaScript for the interactive map.)
What to add, in order of value
Both northern excursions rather than one. On two weeks, Ninh Binh and Sapa are a fork. On three, take both — Ninh Binh and Sapa are each a trip out from Hanoi by road, with the bay cruise as a third. That is the biggest single gain and it is why the north grows most.
The Mekong properly. A night in Can Tho so you are on the water at dawn for the floating market, which is the version that works, rather than the day circuit out of Ho Chi Minh City.
Hue as its own stop. Two nights, the Imperial City and two tombs, instead of a rushed day trip.
And then one of these
Phu Quoc or Con Dao for a beach ending; Da Lat and the Central Highlands for coffee country and a cooler climate; Ha Giang for the far northern loop, which is the most demanding addition. Pick one, not two — and note that each of these is a substitution rather than an extra. A beach ending replaces the delta days; Ha Giang replaces Sapa and wants more than Sapa's three; Da Lat comes out of the centre or the south. Con Dao is the most expensive in days, because its flights and its minimum stay take four or five.
A shape that works
Days 1 to 4, Hanoi. Days 5 to 6, Ninh Binh. Days 7 to 8, Ha Long or Lan Ha. Days 9 to 11, Sapa, including the road travel each way. Day 12, fly Hanoi to Hue and take the afternoon there. Day 13, Hue. Day 14, train to Da Nang and transfer to Hoi An. Days 15 to 17, Hoi An and the centre. Day 18, fly Da Nang to Ho Chi Minh City and take the afternoon in the city. Day 19, the city. Day 20, down to Can Tho for the night. Day 21, Cai Rang at dawn, back up to Ho Chi Minh City, and out.
That is twenty-one days and the same two internal flights as the fourteen-day shape — Sapa and Ninh Binh are reached by road from Hanoi, and the delta by road from Ho Chi Minh City. Day 21 is the tight one, so check the departure time before committing to the dawn market.
The south at three weeks
On two weeks the south is Ho Chi Minh City and little else. On three it becomes a region: two days in the city for the museums and the markets, two nights in the delta at Can Tho, and either a beach ending or the long way out through Chau Doc if you are continuing to Cambodia.
That last option is worth knowing about. The boat up the Mekong from Chau Doc to Phnom Penh turns the end of a Vietnam trip into the start of another one, and at three weeks there is room for it.
The things three weeks does not fix
The weather, mostly. Vietnam's regions have different wet seasons and three weeks moving through all of them means at least one is likely to be in its difficult months. Three weeks gives you slack to reschedule a cruise or a trek, which two weeks does not, but it does not give you a dry country.
It also does not make the long legs shorter. Sapa is five to six hours from Hanoi each way whatever the length of the trip, and that is a real cost against two days in Ninh Binh.
Pace
The commonest mistake at this length is filling the extra week with more places rather than more time in the same ones. Three weeks at a two-week pace with an extra region is more tiring than two weeks, not less.
Build in at least two unplanned days. On a trip this long they will be used — by weather, by something you did not know existed, or by needing to do nothing for a day.
What three weeks is actually worth
The honest summary: the extra week buys you the north twice over and the delta properly. It does not come with spare days built in — the route above has none. If you want weather contingency, you have to leave a day unbooked in Hanoi and another in Hoi An, which are the two places where an empty day is no hardship. It does not buy you a fourth region or a different country.
If the additions you want are Ha Giang and the Central Highlands rather than Sapa and the delta, that is a different three weeks and a more demanding one, weighted away from the standard route.
Booking order
International tickets first; they set the direction. Then the two internal flights, which are inexpensive and fill at the useful hours. Then the accommodation in the fixed stops, then the cruise, then the Sapa trek. Everything else can be arranged a day or two ahead in the country.
How a trip this length should be sequenced against the seasons in three different regions is a question Vietnam, End to End works through.
FAQ
How does a three-week Vietnam trip differ from two?
Start from the standard two-week route, because three weeks is that plus additions rather than a different plan. Hanoi; a northern excursion; the centre around Hue and Hoi An; Ho Chi Minh City. Open-jaw into Hanoi and out of Ho Chi Minh City, with two internal flights and the Hue to Da Nang leg by train.
What should you add first with a third week?
Both northern excursions rather than one. On two weeks, Ninh Binh and Sapa are a fork. On three, take both — Ninh Binh and Sapa are each a trip out from Hanoi by road, with the bay cruise as a third. That is the biggest single gain and it is why the north grows most.
What does the south become on three weeks?
Days 1 to 4, Hanoi. Days 5 to 6, Ninh Binh. Days 7 to 8, Ha Long or Lan Ha. Days 9 to 11, Sapa, including the road travel each way. Day 12, fly Hanoi to Hue and take the afternoon there. Day 13, Hue. Day 14, train to Da Nang and transfer to Hoi An. Days 15 to 17, Hoi An and the centre. Day 18, fly Da Nang to Ho Chi Minh City and take…