Book a longer connection.

Even without checked luggage, you run a big risk of a misconnect. Jetstar certainly has a better on-time record than FD, but the risk is there. Air Asia is pretty sticky about their rules in respect to check-in. You may also have a long hike through a linear airport to connect. The worst case would be a bus gate from Saigon and another to Phuket on opposite ends of the approximately 1 mile long terminal. Although FD departures are usually from a gate, you still have to clear security domestically.

If you have baggage, I will bet you a 500 rial bill that you do not make it.