yt,
if it's widely accepted in both countries?
Never mind Apple Pay, even credit cards are not widely accepted.
The last time I was in Saigon was October. I don't recall ever seeing a terminal that would accept "tap" payments. Most credit card transactions use the old-fashioned paper method. Even when entering my PIN number into a terminal, I usually have to sign the credit card slip. Often two copies - "one for the bank". Don't worry, it's not a scam. As I mentioned before, I only use credit cards in hotels and tourist-oriented restaurants and bars. I expect up-market stores and shops will accept then as well, usually with a 3% commission.
I stick to Mai Linh or Vinasun taxis and never had a problem.
I agree - only use these two companies, but watch out for fakes - I once got into a "Vinasum" taxi by mistake! I noticed my error, when the taximeter had clocked up several km of fares in 200 m of travel! I got out immediately.
Note that the taxi meter shows fares that appear to be dollars, but are in dong - $10.000 on the meter is 10,000 VND.
These days I use Grab taxis (like Uber) in Saigon, but it's hardly worth setting up the app, if you are on a tour.