The Vietnamese are used to European styles and western dress has been adopted this meansthat travelers can dress as they would in the west when visiting restaurants or the bars of the main cities.
Ho Chi Minh is pretty westernized and there are lots of students on gap years as well as bank workers from the west living out there, so really what you would wear in any western city will fit in. There really isn't the big need to cover up that there is in the rest of Vietnam.
As a general rule though the North is noticeably more conservative than the South. The basic rule of thumb is not to wear revealing clothing at any time.
During most times of the year but particularly during our summer months, The Mekong Delta region can be plagued by mosquitoes so long sleeves should be worn from dusk to avoid being bitten.
Clothes are really cheap here so you don't really need to pack too much and its a great excuse to go shopping when you arrive.
For the Vietnamese, appearance is very important, so if you are dealing with an official of any rank, make sure you are dressed smartly.
Do not wear large amounts of jewellery – the Vietnamese consider it impolite to flaunt wealth in public.
Wear particularly conservative clothing if you visit a culturally sensitive area such as a temple or pagoda – generally, the less bare skin the better and specifically, no shorts, dresses, skirts or tops with low neck lines and bare shoulders.
It's really is important to pack for almost all weather conditions.