I did some limited travel around New Year in 2015. Bus companies reduce their services and for days service only major routes like PP-SHV, PP-SR, and PP- Battambang. Reservations for at least up to one week (and probably also more) were available. I found out when I asked about a ticket and was told that between Monday and Friday I had no chance but could purchase the ticket for after that date. Purchase as doing it in person at the ticket counter.
Also, the steep increase in prices lasted a few days longer than the holiday.
having said that even a 50-100% increase should not stop you from your trip.
In sum, the SHV-PP ticket will be doable if you're flexible.
An alternative might be the van from SHV to the Vietnamese border near Kep. Cross there and take a look at the deep south on your way to HCMC.
If you can avoid it, dont travel during those 7-10 days.