In Vientiane, concerning the helmet, you could try Papaya Spa. The owner, serious off road rider,sells Shoei helmets... Not sure which models though. He writes on this board under the name Jean-Louis Papaya.He will be able to tell you if you can find body armour, etc.
Email:[email protected] www.papayaspa.com
Just to say that I dropped by Jean-Louis' establishment today.
He has some very tasty Shark Enduro/MX helmets [no visor] for sale @ $236. He is an official Shark dealer and has other full-face models on sale as well. Best to email him to check on choice & size - Shark [according to J-L] has a different sizing scale compared to other Japanese makes.
J-L does not stock other kit - like armour, jackets, boots etc.
Best to bring your own.
Fast Corner/Paddock/Mr T in BKK have a selection of good gear [tho pricey] if you need to get good gear sourced in the region. You may want to consider routing thru BKK as you cannot rely on picking up good gear on arrival in Vtne or Hanoi.