Another late answer of me, for future readers.
Hoi An to Ho Chi Minh City (depending on the itinerary, 830 km to 950 km along the coast) is roughly half of the "classic" Hanoi to HCMC, a motorbiking route attracting every year so many bikers, backpackers and adventurers from all around the world. Although it is theoretically a very long and dangerous route, many naive or unexperienced travellers approach it as well.
Three weeks in February-March would be magnificent to organize and carry on the expedition but, beware, two people on one motorbike for several days are a key matter to consider carefully.
Three days before this question was originally posted, I was riding a borrowed "Chinese Honda Win" not on the route from the North to the South of the country, but anyway thru some 3 km-long bridges around the Mekong delta, I had a passenger on my back seat, I somehow survived
;) and I have many things to say about the topic.
Lifan is one of the countless Chinese and Vietnamese motor companies or brands that, although unknown in the western world, contributed to the myth of Honda Win. Other names or labels I documented include Detech, Esperow, Ferroli Japan, Indonesia, Landa, Mekado, Sufat, Thailand, Transinco, Vecstar.
For years, now decades, these companies have reproduced always the same design and engine architecture of the model Win that Honda produced until year 2000 (equipped with 110cc engine) and until year 2005 (equipped with 100cc engine).
The common denominator of all of these cheap manual gear bikes is the low quality of the drum brakes and electric system and instruments: these parts get worn out first and others are very often in a bad state on bikes for rent and n-th hand sale.
Also "original" bikes are often fixed and maintained with low quality components, losing their original quality and safety.
Vietnam is long and narrow, so its routes, a 1000cc engine is definitely not needed, but I would never save money on brakes!
Rent vs. buying. I have heard lots of estimations of the cash flow due to buying, using and reselling
Honda Wins but I have no direct experience. To buy, you should be able to rate what you are offered; to rent it is enough to find a reliable company instead.
Definitely, some local companies offer rent plus maintenance or assistence in case of breakdown so this should be an advantage. Some of these businesses have brances in HCM, Da Nang and Hanoi (or anyway in the north, center and south) in order to reach the customers more quickly, if necessary.
Lifan vs. Honda. Still not sure? Honda. Or Yamaha, or any other valuable bike.
Scooter vs. manual. Big matter. A matter of own preference, but mind the local market. Manual gearing bikes like the real Honda Win, or any Honda, or Thai manufactured GPX Legend 200, are popular. Scooters with automatic transmission in a range of displacements are also widely employed.
A third choice, to be also considered to travel out of a city is an underbone scooter. Honda Wave 100cc, or 110cc or 125cc (semi-automatic), Yamaha Exciter 135cc (most likely manual), Suzuki Raider R150. Exciter has a large and really comfortable seat.
Street vs. enduro. Off-road is not really a part of the plan. Again it depends on your taste but for two of you (plus possibly a minimal luggage) an enduro is most likeky not suitable.
Price considerations. Aka 2-stroke vs. 4-stroke.
Honda Wins drink like rockstars and in general two-stroke bikes also need oil. A four-stroke engine may be more economic and also suitable for longer distances (although we're here light years from a "Rome to Milan in four hours" thing).
Road conditions. Follows the parentheses just above. Most of routes in Vietnam look way better than in other countries of SE Asia but, nevertheless, the commercial speed of public and private buses and trains is so low that an additional percentage of travelers opt for motorbikes and scooters. Highways and minor roads are often in a good state but forget to cover 80 km every net hour. Furthermore, one rule holds here more than ever: always be careful when driving!!
Local habits. While driving in Ben Tre, I once saw in the distance an old lady with -
most possibly - her nephew. I suddenly got concerned about their safety and started to decelerate, then to properly brake. All the people around, including the lady and the kid, got really puzzled: I was somehow not supposed to make the two people cross first.
My mindset is to always give way when entering in a roundabout, but this also does not apply when exploring Vietnam.
Other drivers play the hun to say "Hey, beware, I'm here behind you!", although you have wing mirrors on very probably.
I now omit more anecdotes but I could go on for long. In Vietnam they drive on the right side of the road but that is not crucial, no matter what side you do in your home country. When you first try driving there, then in a few miles you just understand the flow of it. Very different from my habit...but it works.
Honda Wins, no Lifan. If you're not a top fan of enduros, then you have more suitable options than enduros.
My very personal suggestion is to either go for a Honda 125cc motorbike or for an Exciter 135 (scooter, four-stroke), more recent, reliable and comfortable.
Rent instead of buying and reselling if you're not too much into the subject.
This article is a must-read about Chinese Honda Win but this other article that I found today compares Chinese with original.
Always be prudent when driving, cruise nicely and enjoy Vietnam!!