Finances: Recently certain posts have reported Hanoi approving DTVs with 500,000 THB available for as less as 10 days. While what's recommended is to have seasoned funds
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months based on requirements of embassy to show financial stability.
Hanoi: Usually has good reviews provided you are eligible and fill up the form well.
Date of application: Recommended date of applying should be < 3 months from the intended date of travel.
Date of soft power activity: Has to be post the intended date of travel. Many schools are willing to revise the letter in case of a rejection or cancellation but this needs to be discussed beforehand.
I hope this helps you!