I know many people who work, or worked for Apple on the hardware side.
I assure you, Apple has "plan B," and it been trying executing on it relentlessly for the last 2 years — just without any success.
Vietnam's total electronics industry output is like a single district in Dongguan. The supply chain is very, very immature there, despite it already towering above any other place in developing Asia, but China.
If what my buddies tell me of Apple's internal assesment of countries is correct, no other countries are even close to a 2nd place alternative on that, except for Taiwan, which is their "plan C" — a sure to work, but expensive option if everything else fails.