We're in Vietnam now and have successfully used Grab both to order food and to use a car. We had a challenge in Da Nang where we couldn't bring Grab up. We had a strong cellular connection but not wifi. Fortunately we were able to grab taxis at a cost that was several times the cost of Grab. But paying $8 instead of $2 (for example) is not a bad thing when it's raining and we'd like to move on.