Say 'meter' (mét) or 'theo đồng hồ' (by the meter) when getting in. Refuse flat-rate offers, which are usually inflated.
The meter starts around 10,000–15,000 VND. A 5km trip in central Hanoi should cost 60,000–80,000 VND. Anything dramatically higher is a scam.
Grab eliminates this entirely — the app shows the fare upfront. Use it whenever data is available.