Hi Raghavi, For travelling from India to Vietnam you need a pre approval letter to get a visa-on-arrival when you get there. You can get pre-approval letter for Vietnam here as it is cheapest and reliable- https://www.myvietnamvisapro.com/
You get Visa-on-arrival for both Cambodia and Thailand. And if you do go on this epic trip, don't forget to share your marvellous travel story with us, when you are back. We would love to read! Write here- https://www.tripoto.com/trips/create Keep exploring!
You can apply online if it is visa on arrival or you can get when you reach at the airport. Remember to carry your photos and the currency of their country. If visa on arrival not available than you have to go Visa office near you.