Hey Wanderir,
As Rod has already mentioned, it looks like you are doing your research, good move, so you are well on your way to organising a sucessfull trip.
Its tempting to try and 'do it all ' and sometimes that results in riding everyday and not allowing time to stop and take in the scenery, so first tip would be to get a 3 month visa, a bit more expensive,but gives you the flexibility to chill every now when some amazing experience crops up, and lets face it but by the time you have cruised around the madness that is Hanoi, found a bike ,(good luck with that,) got it ride ready, well, theres a week gone at least,then there is Halong Bay a few day detour, the northwest loop, another week if you don't do any trekking out of Sapa, and then the trip south, as you know road conditions aren't what we are used to in the west, 200 km a day can be a major undertaking on an underpowered or clapped out 125cc road bike on some of the worst, but by the same token most amazing, roads in SEA,
The trip south could take anything from a couple of weeks to a month, alternating between the notorious highway 1 which follows the coast, and the Ho Chi Minh highway which winds its way through the mountains,(but if you want the HCM trail, get a dirt bike and head to Laos, but thats a different story.) My mate Shing and I were generally taking in the sights and visiting places of interest and memorials from the American war, which took us on and off the main tourist routes, but gave us a great insight into the country and a bit of its history.
From Nha trang a bit of a detour via the central highlands around Dalat is worth a week of anyones time, and then there's Saigon to explore, at least another week, the Meekong delta a few more days, throw in a couple of breakdowns, a romantic interlude and plenty of cheap beers and theres six / eight weeks of anyones time, i spent 3 months in total in Vietnam but that involved too many of them cheap beers.
The border crossing into Cambodia should be straightforward, depends what you read, and what day it is, but i wouldn't do it on the last day of my visa just in case the bike didn't get through cos the papers arent in your name.
There is a better choice of bikes available in Cambodia, so i would sell the bike to someone going north from Saigon , jump on a bus and rent a dirt bike in PP.
Time and a few dollars means you can be flexible in your plans, lots of factors effect your daily decisions,the weather, the people you meet, quality of the food and accomodation available, etc etc.
I didn't have any dealings with the guys at Flamingo travel, and they could be an exception, but i like to be able to inspect any bike i am thinking of renting or buying before i part with any of my hard earned dollars.Due to government restrictions anything over 125cc is usually illegally imported into vietnam and may have dodgy paperwork as a consequence,having said that, it was a year since i did my trip and things change daily, there is no substitute for being there to know the true story.(but never believe a salesman, wherever he comes from.)
As for the insurance info keep it on your person, the bike and helmet may not make it to the doctors with you, and if you are not near a decent sized town then in the first instance, local care is the best you can hope for. Thats the risk of riding in SEA, my advice would be find someone else to ride with so you can look out for each other, there are plenty of people doing that trip, so shouldnt be too hard to find.
last tip, try to learn a bit of the language, away from the major cities, especially in the north it can be hard to find an english speaker, so at least get a phrase book and learn to do charades.
I hope that helps, have a great trip, stay safe , but most of all, enjoy
Hudson