I don't really agree with this - especially the "don't argue with anyone" bit. If you have been "done" then by all means argue - of course language can be a barrier. In the case of taxis, a good way is to get your hotel to argue on your arrival if that's where you're headed. Just don't take any risks with personal safety - and I accept those risks can be difficult to assess.

Based on disputes I've intervened in, half the time its a genuine misunderstanding made worse by the foreigner arguing away with an attitude that everyone should understand English. If you're in Vietnam without speaking their language then you must be prepared to give some lattitude.

Nobody wants to involve the police - so threatening to call them can have results. The police run their scams to get their income... but that's probably not going to be their first thought if you're the genuine victim of a crime and the police become involved. Saigon police stations are supposed to have at least one English speaker available. My experience is that they do and that they have English versions of any witness forms etc that you might be asked to fill in. On the other hand if you start spouting about the brand of passport you own, things are not going to go well for you.

If the taxi is "legitimate" but the driver is the problem then call/threaten to call the taxi company. I was once told that so many Saigonese want to be taxi drivers with the legit companies that if they receive as much as one complaint from a foreigner about one of their drivers they lose their job. Probably not true - but they've been pretty worried when I've got my mobile out.

Some companies have a label in English and Vietnamese that clearly states "no meter, no fare" - ie you don't pay. Of course sometimes they forget, so don't be mean about it if without the meter running he asks you what you'd think would be a fair price anyway.

The tip about the broken meter cover is a good one. In general its best to flag down a passing taxi on a main road or get to know where the taxi companies have their ranks. Learn to recognise the companies by their colour and phone number. The yellow Vinataxis (8 111 111) are probably the easiest to spot. Some scam drivers copy popular logos, but its rare for them to have the same phone number displayed.

An "odd" tip I would add is once you've mastered this art, never allow a VN friend to flag down a taxi unless they are travelling with you. Most VN people rarely use taxis and seem to care little who they travel with. Some will negotiate the fare anyway rather than use the meter. If they get a taxi for you they are likely just to wave at the first one they see, regardless of the company.

Finally, for anyone who is in Vietnam for more than a few days, its well worth getting a local pay-as-you-go mobile SIM card. You'll need a GSM900 phone that isn't locked to a particular network.