When we booked our tickets for our Vietnam – Cambodia trip, we already decided that we were doing a cross-country trip. We booked a ticket from Manila to Ho Chih Minh then Siem Reap to Manila. We had no choice but to do our research on how to get from Ho Chih Minh to Siem Reap. You are probably reading this because you, too, are planning to do a cross-country trip from Vietnam to Cambodia.
It is very easy, actually.