I rode Bangkok to Saigon through Dec-Jan. 1,500 of mainly flat kms over 19 lazy riding days. Around $10 a day for food, and $20 on average for guesthouse accommodation. Great countries, lovely people, safe, insight-full. Ride and route details, photos, accomodation links, etc. on http://bangkoktosaigon.wordpress.com/.
Happy to answer any questions if you're thinking of doing a similar trip (or even if you're not!).