Vietnam
Best when you give it enough time. Start with route shape, budget cautions, transport notes and what not to cram into two weeks.
Countries
Start with Thailand and Vietnam, then choose a route you can actually travel.
A good country page should help you choose a first base, a realistic pace and the places worth slowing down for.
I am focusing this guide on Thailand and Vietnam first, so the core country hubs can become useful before the map gets wider.
Thailand
The easiest place to start, if you do not rush it. Good for food, islands, northern routes and learning how the region works.