How to use ride hailing apps (Grab, Uber, etc.) in Vietnam
Ride hailing apps (or taxi hailing, ride sharing, whatever you call them) are used everywhere in all major Vietnamese cities. In big cities like Vietnam and Ho Chi Minh City, they're the best way to get around because they're both cheap and readily available
The most popular app is Grab (http://grab.com), which I personally recommend. Uber doesn't operate in Vietnam after it sold its entire business there to Grab in 2018. There are lots of other apps (like Go-Viet, Vato, etc), but from what I've seen there most visitors stick to Grab because it has the biggest coverage and highest density of drivers in the country. Plus it's based in Singapore and English is the default language used on the app and you can communicate with drivers and support team in English
These apps are completely legal too, and you can use them anywhere including the airport.
Prices: the prices are generally very low, with most rides within cities costing no more than $3-$4USD. Additionally, in Vietnam you also have the cheap motorbike option (instead of a car), which for most rides will come out to just $1-$2 going from pretty much anywhere in the cities. Here's a snapshot I took when I was in Hanoi on my Grab app, going from one end of the city to pretty much the other end of the city. The currency is VND (which is worth 1/23,000 of a USD)
How to install the apps: install them normally from your phone's app store. My only advice here is to download and set up these apps before you leave your country. The reason is that these apps (at least Grab, which is what I used) will send you a SMS text message to confirm your phone number during the setup process, so you'll want to do it somewhere you can receive these text messages. Once it's set up, you can swap out your SIM card and the app will still work fine, so getting a local SIM card in Vietnam after you arrive won't affect what you've already set up
Payment method: at least with Grab, you can connect your credit card (Visa, MasterCard, American Express, and JCB are the only ones accepted) and pay directly from your card, even for tiny amounts like $1-$2. Grab also lets you pay cash directly to the driver too, but I recommend using credit card to avoid having to fumble with cash
How to use the apps: Grab works exactly like Uber. Just open up the app while you have internet, pick a car type, enter your destination (or drop a pin on the map), and then call a car/bike to your location. Obviously because it requires internet connection, you need to have access to wifi, or better yet, a local Vietnamese SIM card so you can get data on the go. You can use the chat function in the app to communicate with your driver, and the app will automatically translate English into Vietnamese and vice versa, so you don't have to worry about language barrier
Safety: I found Grab to be pretty safe overall. Didn't have any sketchy drivers with any of the car types I used, and obviously because the entire ride is tracked and price is set upfront, there's very little risk of scam. If you order the bike option, all Grab drivers will give you a helmet to wear, which is nice. I'm not sure if other apps will give you a helmet. Definitely don't ride a motorbike in Vietnam without wearing a helmet, the traffic is absolutely bonkers there!