Hi Su, If you have single entry visa you can travel to Hanoi in north Vietnam from Mumbai then travel down south till Ho chi minh city by bus or flight (they are cheap from hanoi to HCMC) . You can then take a bus into Phnom Penh in Cambodia from there. And then you can enter Laos from Cambodia overland or by air and take a flight back to Mumbai from Luang Prabang after exploring the place. You can check the itinerary or travellers who have travelled to Vietnam, Cambodia and Laos here: https://goo.gl/1qpVe2 Thanks
Hi there are several operators in Vietnam offering cruise in halong bay in money ranging from $100-$300. You will come across several when you are in old quater of Hanoi. I never took a cab! Read about my travel here: https://www.tripoto.com/trip/the-story-of-my-40-ride-across-vietnam-1-motorbike-1-girl-58474003118c3