If you’re traveling in the school summer holidays you will be very much off season for India and Sri Lanka - it will be monsoon season. Can you not go at any other time? While Monsoon season isn’t so bad in some places, it’s really not when you want to be considering India. Kerala is the place I was going to recommend to you - gorgeous place, interesting history, extremely safe, beautiful, amazing food. Look up Munnar, Aleppey, Fort Kochi, Varkala, Cherai - and I was going to suggest that you visit Kerala and Goa on the same trip. For Goa I’d recommend Palolem beach for you. However do not go during the summer holidays!! Goa is pretty much totally shut down at this time - a lot of the lovely beach huts which house the businesses (little bungalows and cafes, bars and restaurants) are just totally dismantled, ready to be built again later after monsoon season. The atmosphere is quite depressing, to be frank. The only visitors are those who booked by accident not realizing it would be monsoon season. As for Kerala, don’t even consider it during UK summer holidays. It can flood and again, it just won’t be a nice holiday environment.
However - if you want to go at another time of year - I can’t recommend these two places enough! Kerala is like another world compared to the rest of India. It’s fresh, green, beautiful - they call it ‘God’s own country’. It’s really safe and easy to travel and you don’t get any of the scams you might meet in the rest of India. Palolem beach is Goa is also wonderful. Just make sure you go at the right time of year.
If you want to go to Vietnam I’d recommend Hoi An, Ho Chi Minh, Ha Noi and Ha Long Bay. Again, the UK summer holidays are not the best time to visit Asia so be prepared for the weather to be less than ideal.