Apply in korea now. Why wait for vietnam? It's already backlogged there and taking longer than usual to process.
I've gotten mine from Vietnam with 6 months bank statement and my friend got theirs with 1 month bank statement. Approval depends on how fussy the person assessing is to be honest. It's so inconsistent.
I didn't have a job when I applied and they never asked for proof of income. Where as my same friend had to show tax return and write a letter he he recently quit his job to travel