Vietnam transit visa

I am travelling from Siam Reap to Sydney, with a connecting Vietnam Airlines flight from Saigon. I will not be leaving Saigon airport as there is only 4 hours between flights. I have been in Vietnam for the past 3 months but my current visa will have already expired when Itransit through Saigon. So, do I need a visa for these few hours at Saigon airport going from international flight to international flight ie. no domestic flights involved.

Michael