Competing for the allegiance of Southeast Asian states is important for India to keep China from gaining a foothold on the Bay of Bengal. Now the Chinese would have to run a gauntlet under the yes of Indian recon based in the Andaman and Nicobar Islands. Burma (Myanmar) is a key prize in this contest.
When Burma was closely aligned with China, I speculated that the major role the Indian army could play in a war with China would be to invade Burma to remove any Chinese bases there. Now Burma is in play, it seems. Which means the Indian army might have a role in helping Burma defend itself should China try to drive to the Bay of Bengal. Japan tried that route to India in World War II.
Another objective of India would be to attrite the Chinese fleet as it sails to the Indian Ocean. This is where Vietnam comes in. From my Jane's email updates:
The Indian Navy (IN) will train 500 Vietnamese submariners in underwater warfare as part of the expanded strategic and defence ties between the two countries announced in New Delhi on 20 November. "India will continue to assist Vietnam in modernisation and training of its defence and security forces, including through the USD100 million line of credit for defence purchases," Indian Prime Minister Manmohan Singh said, during the visit by Vietnam Communist Party general secretary Nguyen Phu Trong to New Delhi.
India has been interested in closer ties with Vietnam for some time now. Vietnam actually came out on the good side of a short war with China rather than receiving a drubbing as India experienced. I have no doubt that Vietnam's ground force capability has eroded relative to China's over the last three decades, but Vietnam would still be an asset from geography alone.
The ability of Indian subs to seek shelter in Vietnamese ports would help India, too.
India could really expand this ability to hit the Chinese navy in the South China Sea by establishing an air expeditionary wing of fighter, recon, refueling, and naval strike aircraft. If India could send such a unit to Vietnam (or Thailand, Cambodia, Malaya, Singapore, or Indonesia for that matter) at the outbreak of war, India could give China great headaches.
Since China's anti-ship ballistic missiles can already reach the Bay of Bengal, fair is fair.