Yes, a visa would not be required provided you don't need to check in the baggage or leave the transit area. I have never explored this option as it seemed risky, but are they liable for any loss of baggage during baggage transfer? I was also told long ago that Airlines of the same group(Star Alliance, One World,etc.) and partner airlines have baggage transfer agreements. Vistara and SQ are partner airlines, so it should be possible. Maybe I'll try this when I'm flying solo.
Regarding the previous post, I feel it is cheaper to book two one-way tickets between IN and VNM. It would be 40K miles+tax, but 46.9k+tax if done through SE program.