Why "sau" instead of "sau khi"?
khi is unnecessary here. Sau khi is used prior to you mentioning another action. For instance:
Tôi sẽ dọn đi sau khi trở về = I will move out after I return.
Thank you. Now I get it. "Sau" is a preposition, and "sau khi" is a conjunction.
Is it actually wrong to use "của" before "Việt Nam" here?
The English sentence should start with "The". It is not correct without it.