After the Treaty of Amiens, Decaen was made captain general of Eastern colonies, and Vandermaesen was chosen to second him. He embarked at Brest on 15 ventôse an XI (6 March 1803), bound for Pondicherry, but with another war with England breaking out, the French division diverted to Isle de France (Mauritius). Vandermaesen was put in charge of the division, and was promoted in the Legion of Honour on 5 germinal an XII (26 March 1804); as second to the governor of Mauritius, he commanded the troops garrisoned there and in Isle Bourbon. After the surrender following the invasion of the isle de France, Vandermaesen returned to France, where he was received by Napoleon, and given command of a division under General of Division Caffarelli.