Cheapest way would be overland to Bangkok then take a cheap Thai AirAsia flight to Hanoi. They do tickets for as little as 1,500B (before taxes and surcharges).
Latest departure from Bangkok is 19:00, so if you left Siem Reap at the crack of dawn, you could feasibly get that flight and be in Hanoi the same evening.
If you go overland through Phnom Penh and Vietnam you're looking at at least two to three days of travel.
#2 somtam2000 has been a member since 21/1/2004. Location: Indonesia. Posts: 7,798
Send somtam2000 a private message Where has somtam2000 been? Website Twitter Facebook Flickr Google+ Instagram Pinterest