Adding an extra trip

My original plan was SYD - TAO - CTU 19th Dec, then return the same way SYD - TAO - CTU 2nd Jan with a 18 hour stoppover in TAO. I have already bought the ticket and obtained a single entry Chinese visa. Now I would like to go to Vietnam, is it possible that after I get to CTU, I will get a return ticket CTU - SGN then come back to CTU on 1st Jan and get out to stay overnight in TAO then back to SYD with a 24hour or 72 hour transit?