"Vietnamese feudal system ends after the second World War."

Translation: Hệ thống phong kiến Việt Nam kết thúc sau chiến tranh thế giới thứ hai.

[MrBallpeen]

Why "sau" instead of "sau khi"?

[TehVanarch]

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.

[MrBallpeen]

Thank you. Now I get it. "Sau" is a preposition, and "sau khi" is a conjunction.

[TehVanarch]

Spot on!

[Dan553966]

Is it actually wrong to use "của" before "Việt Nam" here?

[Seth853809]

The English sentence should start with "The". It is not correct without it.