Con Dao Islands are connected with Vietnam's mainland through flights and ferries.
There are no direct flights from India to Con Dao. Flights can be taken to Ho Chi Minh City and then Vietnam Airlines to Con Dao.
Ferries are available from Vung Tau on the mainland which is around 200 kms Con Son island. It costs around 100,000-200,000 VND. Yes, this cheap! But the journey is quite long and tiring, and it is advisable to opt for a flight journey.