India's foreign policy has made a drastic turnaround since Narendra Modi came to power. It's become bold and decisive. The latest and perhaps the most overt of all decisions is the sale of the BrahMos missile to Vietnam, a nation that has serious issues with China.
Vietnam has confirmed the purchase of the missile. This move will definitely anger Beijing who's been used to taming India into inaction every time India has looked to export weapons to nations that've differences with China. But the Modi government has stood against this irrational belligerence and shown China that India will do what it feels is appropriate for its interests.
The Narendra Modi government has expanded defence cooperation with Vietnam who's a rising power and an archenemy of China. India's then defence minister, Manohar Parrikar, had asked private Indian firms to help upgrade Vietnam's battle tank electronics and to help fit the country's Mi-8/17s fleet with software-defined radios. India may also sell the Akash SAM systems to Vietnam and has begun to train Vietnamese pilots on Sukhoi fighter jets.
This export is historic not only from the viewpoint of geopolitics but also from the fact that this is India's first export of a major offensive weapon to another country. A few other nations like Malaysia and Brazil are also interested in its purchase.
China has for long provided Pakistan with weapons and nuclear technology. It has even gone to the extent of defending Pakistani terrorist Masood Azhar in the UN repeatedly. The only motive behind arming Pakistan is to ensure that India remains occupied on the western border while China can focus on it's maritime strategy and create an efficient and almost insurmountable logistics and military network on its border with India.
Finally, India is giving China a taste of its own medicine. Selling weapons to Vietnam; giving a credit line to Mongolia; running the Chabahar port in Iran; offering to run a Chinese-built airport in Sri Lanka; the Modi government is doing it's best in tackling the growing threat of China and its rogue partners.