You're right but it's merely a case of a nuance in Vietnamese. Words ending in -ong/ông/ung force you to bind your lips together/close your mouth. This, therefore, makes your mouth end up in a position similar to that of -m. Ong sounds like ongm as a result. The same thing happens when you say words ending in -oc/ôc/uc but in those cases the resulting sound is more like a -p. Tóc sounds like tócp.
No he's got a point. The -ng in ong is not the usual -ng sound. This is because words ending in -ong, -ông and -ung force you to close your mouth as you say them. The same thing happens with words ending in -oc, -ôc and -uc (except with those, they end up sounding like a -p).
Ong sounds like ongm.
Tóc sounds like tócp.