Try going here
http://www.aleale.com.vn/
Otherwise you can buy tickets on the day of the match outside the stadium, but be prepared to pay twice as much, or more.
I wouldn't put too much hope into it because it's really tight this time...
It might be worth a try to go to the Vietnam Football Federation office, on Ly Van Phuc St , off Nguyen Thai Hoc. It's in a office building on the right about 2 mins walk from the Nguyen Thai Hoc end.
I bought tickets for the Asian Cup last year there when everyone was saying that they were sold out.
