The easiest way to get to Con Dao is by plane from HCMC (8x per day) or Can Tho (1x per day) operated by Vasco Air. You can purchase the tickets from Vietnam Airline’s offices and website. Unfortunately, on occasions tickets are not available online, as several travel agents purchase all tickets. In that case check with an agency or contact Vasco Air.
In July 2016 a new ferry service has started from Soc Trang (Mekong Delta) to Con Son. The Superdong ferry (who operates between Rach Gia and Phu Quoc) leaves Soc Trang at 8am, and Con Son at 1pm. The ferry ride takes around 2 1/2 hours. If you visit the Mekong Delta before or after you stay on Con Dao, this might be an interesting option. For more information visit the official Superdong website.
For the more adventurous people there is also a boat from Vun Tau to Con Dao. However, this ship mainly deliver goods to the island, and the 13 hours journey can be apparently very rough. Also, the ship has no timetable it follows.