@Darren ******
the odd thing is however that Vietnam doesn't require proof of onward travel from visa holders. For the 15 day visa exemption you need it, for anything longer you do not. Vietnam Airlines has been reported by many to wrongly check for proof of onward travel but if you consult the Vietnamese authorities they're quite clear on the matter.
Or were you indeed going all these times with 15 day visa exemptions?
Cambodia requires proof of onward travel only from some nationalities (Ethiopians, Kenians, Zambians, etc.). Entrants from developed countries or from the United States do not need it.