To people have done thier homework like us, we can differentiate Android and Samsung. To those that aren't into tech and just sign any handset plan, Samsung = Android. They don't do research or anything, but take the best-looking phone offered by Samsung. It also helps that Samsung was the successful pioneer among Android brands in Singapore that was fighting with Apple since the days of Froyo. While brands like Sony, HTC and LG were present in SG during the early era of Android, their advertising can't match Samsung.
So Samsung became the flag-bearer of Android and many people ignorant in tech would think Samsung is Android.
As for Vivo, I really wonder what is their niche. Their brother brands have specific niches. Oneplus is focused for Android enthusiasts, Oppo is for those who prefer camera at the expense of OS updates and Realme is essentially Oppo's budget subsidiary. If Vivo doesn't find their niche, they are unlikely going to survive in the years to come.
I would think that iOS may be dominant in the years to come for Singapore. Sometimes, I have to travel to poly and uni to work on projects and I notice many are using Macbooks nowadays. I suppose the rising affluence of the younger people and Apple's successful advertising of itself as a lifestyle is attractive to the younger generations. You always see many with Airpod Pros.
Agree with you that S20 is way not worth it to buy now unless you have some CIS discounts. Even then, for Exynos, it is not bang for bucks