Events in the recent past have made it clear that the Narendra Modi government is serious about India's military and geopolitical ambitions. It is a truism that a blue-water navy that can project power over long distances is a necessity for any major power, especially one with global ambitions. Equally, it is clear that any power worth its salt has to deter other powers from doing things that would hurt its geopolitically.
In this context, Vietnam becomes significant for India for several reasons. It is clear that India's most important enemy is China, which has embarked on a 'string-of-pearls' strategy to surround and contain India in a "South-Asia" ghetto of sorts (incidentally, the Chinese have been the most enthusiastic propagators of that meaningless term in an attempt to dilute India's fairly good and historical brand, while they aggressively push the 'Greater China' brand, notably not an 'East Asia' brand.)
Vietnam is possibly the most potent of China's neighbors in terms of military skills; at any rate it is the only one to have defeated China in battle recently. In 1979, China invaded Vietnam (coincidentally while India's then foreign minister Atal Behari Vajpayee was visiting Beijing, in what amounted to a diplomatic slap in the face for India). But the battle-hardened Vietnamese were more than a match for the Chinese, who were forced to withdraw with a bloody nose.
Secondly, the Vietnamese are currently victimized by Chinese aggression in the South China Sea, which the Chinese consider their private lake, where they have deemed a vague "9-dash line" citing obscure old maps as the limit of their exclusive economic zone. In effect, they claim most of the sea, putting them in conflict with Vietnam, the Philippines, Taiwan (which has historical claims to some of the islands) and everybody else with interests there.