Hopefully you've got enough sense to pack your passport, 'plane tickets and travel insurance... if not then you should think twice before leaving the house let alone cycling in SE Asia. But what else to take?
Clothes
Take at least one long-sleeved top and trousers in case you really need to keep out of the sun and to keep the mosquitoes at bay in the evenings (light colours are supposed to deter them). Otherwise, take as little as possible. If you're planning to go to mountainous areas you'll need a jumper and maybe even a hat and gloves, but remember you can buy (and discard) clothes as you need them. It's nice to have a different pair of shoes for the evenings; lightweight ones like Crocs are ideal. A lightweight raincoat is handy for windy boat rides or cool, rainy days which do happen even in the dry season, particularly in the north of Vietnam.Sunscreen
Good, high SPF sunscreen is generally only available in tourist centres, so stock up whenever possible. The more you cover up, the less sunscreen you need.Medical kit
A travel doctor can give you a kit containing prescription drugs for traveller's diarrhoea, a general purpose anti-biotic, drugs for giardia, drugs for nausea and vomiting, counter drugs for stomach cramps/pain relief and counter drugs for general diarrhoea. Don't forget the basics as well: plasters, bandages, tiger balm, paracetamol, ibuprofen + codeine. Cortisone cream is good for cases of heat rash.Anti-malarial drugs
They're expensive and not good for your health but malaria's pretty nasty too. Doxycycline should be taken 3 days prior and 4 weeks after being in an affected area, for a maximum of 4 months. If you travel in and out of malarial areas you may end up taking it the whole trip. Kat had no side-effects whereas Mat got bloating after a month and a stomach ulcer after two months. Neither of us was aware of photosensitivity.Mosquito net
We had one which took up valuable space in the panniers but didn't use it. If the room needed one, there was usually one provided. When we did want to use it, there weren't hooks to hang it from. Also, because it was a travel net, it was too small to be useful.Silk sleeping bag liners
Taking a mattress with you might be excessive