Vietnamese here. I'm not sure about travelling India -> Thailand, but I can answer the second part. For travelling from Thailand to Cambodia to Vietnam (especially Ho Chi Minh City), you pretty much only need a passport and some money. Buses frequently travel between these areas, so you won't have trouble finding them. My advice would be to pack light and keep your belonging close to you at all times. The buses are often crowded, and it's best to avoid anything gone missing by the time you wake up. Other than that, it's a common route and you won't find much trouble with borders here.