Vietnam Airlines flies both routes direct. Overland you're looking at at least 3 days non-stop travel, so flying is a good option.
#2 somtam2000 has been a member since 21/1/2004. Location: Indonesia. Posts: 7,738
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Vietnam Airlines is the only option. It costs USD $130 unless you are thinking of taking a bus from Phnom Penh to Ho Chi Minh City. A number of bus companies travel from Phnom Penh to HMC. They offer the one way trip for USD $8-12 and it takes approx 6 hrs.
From HMC you can to Hanoi fly one way with Pacfic Airlines starting at USD $70 including all taxes.
#3 ernieh has been a member since 21/3/2007. Posts: 49