The ‘Sinosphere’ is the cultural and linguistic area which has been influenced by China. This area includes the Vietnamese language, which has undergone heavy Chinese influence. At the same time, Vietnamese is an Austroasiatic language by descent, related to other Austroasiatic languages like Khmer (as you note), Mon and Santhali. Most other Austronesian languages are not part of the Sinosphere, but Vietnamese is.