If you tell people who live in the ratified atmosphere of the "Stepford Wives" area called Phu My Hung, this type of thing doesn't happen.
In actual fact, if you read the VN newspapers (Cong An is a great one for the latest poop) you will find that PMH is neither safe nor so nice.
For example I was driving through Q4 last week and nearly whacked a guy with my motor-scooter who was standing in the middle of the road with no pants on. Not very usual but the local population didn't seem too put out by it.
The JV that set up PMH also has it's own security outfit that drives around 24/7. Obviously none too effective.
You have to view al this from a VN perspective. The lack of public toilets, except in Q1, means that it is not uncommon to see males and females relieving themselves. It might not be too common in your home country but here it is the norm.
PMH is plain dangerous at night. (I travel in to Q7 every morning between 02.30H and 04.00H every day I am in TP HCM). Large swathes of the area are wide open to crime. Motorcycles stolen from outside stores, even from riders (after they were pushed off).
There have been several complaints of flat out sexual assaults (breast pinching) in PMH, except they like to say it happened in Tan Phu Ward next door.
There are many reports, in the VN press, of iPhone snatching in Q7,if you have to make a call either duck into a store or lean against a pole (with the fruity thing to your ear nearest the pole). Don't flash your 'Bling'.
Always be aware of the situation around you, trouble always has tell-tale signs.
Don't use Xe Om after 22.00H, they often go astray through high-risk areas. Don't use pirate taxi's, tell or signal to the taxi driver which way to go (and don't think he is going too protect you).
Always take cab decal and plate numbers - a French TV director was complaining in PMH Neighbours Group that a cab driver was allegedly masturbating as he drove her to PMH (he most likely had an itch from sitting all day, of course). She never took his number or even complained!
The area from Khanh Hoi (Q4), past Lotte, is a high risk area. Robbery, assault, motor-scooter theft, etc are de rigeur for the area behind Lotte.
You said the Cong An didn't react. If you reported it at night, you were lucky they were even awake. The smarter Cong An usually work days and your chance of an English speaking one is greater.
The office where you register your occupation of accommodation usually have English speaking people available - they even have laptops and use Google Translate to communicate with you.
One thing makes anything work in government here is paper. Sit down at your computer and write down your complaint in short sentences, each separated by a double return (enter). (Google translates short sentences better)
Copy the whole complaint in to Google Translate then copy and paste Google's version of Vietnamese above your English version and print it out and deliver to the local Cong An.
Put on your copy the fact you are giving a copy to Q7 Cong An headquarters. This will cause them to do something, even if it doesn't satisfy you.
Remember, the Cong An regard Foreigners as a privileged class who get away with murder (literally) so there is a certain amount of resentment. So this will also decrease the likelihood of anything happening.
Flashing is not a serious crime, sexual assault/contact is.
The consulate people were most likely flapping their lips, but might have updated their 'post' advice, but most likely they never even made any move outside their compound.
Remember, diplomats have to go through 'channels' and complaints such as yours most likely got 'binned' as it could be seen as interference with internal affairs of VN.
Carry your camera, you will get a memento for you and the police! The fact he ran away suggests he is a harmless flasher rather than a potential attacker.
And it simply proves that the added costs of living in 'Stepford' doesn't make the area immune form anything.
(You omitted from detailing the street where the incident happened so others could avoid the situation)