I haven't used Grab in Vietnam but I use it often in Thailand. It's about the same as Uber. Sometimes it's cheaper, sometimes it isn't. Reliability seems about the same, quality is about the same. I generally compare the two fares and go with the one that has the least amount of wait time.

Edit to add: In Asia, the fare difference is generally going to be measured in cents, not dollars (or euros, pounds, whatever.)