Personally, I never win at the price waiting game. For my trips, it seems like the price usually go up as I get closer to the departure date.
I just pay the price if it is reasonable to me.
I checked a mid-week in June for one-way for SGN-PQC, HAN-DAD, DAD-SGN on Vietnam Airlines website. The coach ticket prices I saw for each of these segments range from 58 USD to 72 USD. The trip duration is between 1 hour to 1.5 hour. For me, in these cases, the price is certainly reasonable and I will buy them now.
I always stick with Vietnam Airlines by the way. I do not have any experiences with the other local airlines but I prefer to stay with VNA.