I recently did a mileage run to Vietnam and used Uber via WiFi there several times. I thought of getting a local SIM card, but ended up not.

The only time I ran into problems was leaving my hotel back to the airport, the driver seemed to think the hotel entrance was on the street behind it (I was putting in the hotel's name as my pickup). After a few tries and the same driver picking up my fare, I finally just put the hotel's address in...and driver was there in a few minutes (but I did cut things a little close for airport).

Otherwise I had no issues with just using WiFi for Uber. From hotel, I could request from hotel lobby and the WiFi even worked outside. Otherwise, there were enough coffee shops, bars, restaurants, etc. where I was able to poach WiFi.
Outside one museum I couldn't, but there were people selling drinks and stuff, I just asked, "Hotspot?" and one of them gave me a hotspot to his phone so I could do an Uber.