Jimoi,
During my visit last month to loas from Huai Xai,i meet david at Changmai ,thanks david for info and buy a loas map from him....
I already have are ambesy latter from loas ambesy at kuala lumpur..
so went i entre huayXai fromChiangKong..the custom and police just sign and stamping..at behind the latter...no paper given as they say.. i heard and understand just vip..
we just buy a insurance as recommed by David.and during
out from loas at vte we just show the paper..the custom say ok. u can go.so we hadeding to NongKai.
before my trip, my friend entre vte with bike thai custom already remain him u cannot entre loas at vte with bike..my friends say he have a latter from loas ambesy..Thai custom say Good Luck.so at vte customer he go to room 5 and get everything settle between 30 min..so he entrevte with bike and out from loas at Huai Xai.
and at luang lamtha i meet a loas reporter/tourism..i just say we entre loas fro Huai Xai and we have difficulity entre at vte..he say we can entre but we must get a permit..(What i think we must get latter from ambsy)..
then he say thanks for coming to loas and support their country on tourism.

thats my experience entring loas.