One difference is the level of aspiration - English /d/ is much more heavily aspirated than Vietnamese /d/. Put your hand (or a sheet of paper) in front of your mouth when you say English "do" and Vietnamese "đu"
This distinction is even more important when it comes to the voiceless counterpart "t" - initial English "t" is very heavily aspirated while Vietnamese "t" is not - compare English "tea" to Vietnamese "ty". In fact, if you aspirate the Vietnamese initial "t" it sounds like "the" (thi instead of ty).