I sent the laptop to Asus repair, turns out they sent it to a random computer repair shop, not some Asus facility, and the repair person did nothing as it passed the tests so its not "broken".

I then got the laptop back and had bought myself an ethernet adapter, so was able to try ethernet, and it works perfectly on ethernet. No jittery ping, as good or better than my old laptop was on wifi.

Then if I switched to wifi, instantly terrible ping result.

I dont know if the wifi adapter is defective, or just excessively sub-par.

I am so satisfied with the laptop now, but tragically I found another great laptop deal a few days ago and bought it. Its much more expensive but specs are 2x as good and better in a lot of ways. So I will be returning this vivobook after I have received and checked out the new one I bought.