For VN domestic flights there is no reason to purchase expensive fares now. 28 Dec. should not be that busy and the fares are likely to drop closer to departure + if you you purchase the ticket now it is very likely that you will have a 1 or more schedule changes. There is nothing to worry about. If you miss the flight to PQC just pay to get rebooked to this next flight - change fees in VN are modest and so will the upfare be.

Anyway, to answer your question: 2:15min should be more than sufficient to clear immigration and head for the dom. terminal (5 min walking) for check-in.