How many days in advance should I prepare my Soft Power Application for submission in Hanoi?

So,

1. first you need to prepare the documents

2. Full up the application form and submit

3. Pay for the application and wait

You are allowed to apply and submit only once you are in the jurisdiction area of the Embassy you have picked ie in your case once you are in Hanoi.

From submission/payment to decision can be anywhere between 4-12 working days in Hanoi. It varies from case to case.