Do you have a route planned in Lao yet?
From China you will enter at Boten. A short detour to Luang Nam Tha and Muang Sing is worth it, if you think youu will have time. Otherwise I imagine you will head for UdomXai. From there you have a few choices - if you want to go to Luang Prabang there is the standard H13, but I reckon ,most of that road is pretty tedious. An interesting and rarely used route is cycling to Pakbeng on the Mekong and then getting a tourist boat to LP.
A different experience is to cycle on the beautiful little road from UdomXai to Muang Khua on the Nam Ou river, then boat to Nong Khiaw. This trip isn't that cheap as it is mainly a tourist run nowadays, but great scenery. Just riding to Muang Khua with an overnight (leave most stuff in UdomXai) is worth a go. Alternatively you can cycle to Nong Khiaw from UdomXai. From Nong Khiaw tourist boat or cycle to LP or the start of route below.
For me the best route in the area is to get to Nong Khiaw by whatever means and then there is an excellent route along H1 via Vieng Kham and Vieng Thong. It is fairly tough and very remote, but all paved and well worth the effort. When you reackh H 6 you can go to Sam Neaua for the caves at Vieng Xai (Pathet Lao HQ in American War, well worth a visit). Might be best to bus back if you don't want to repeat ride. Then along H 6 to Phonsavan (Plain of Jars and more UXO than you want to see in a lifetime).
From Phonsovan, carry on the wonderful H6 to H13 and on to Vang Vien and Vientianne. Alternatively try the more recently opened road to Paksong. Not ridden that one yet, but someone here did recently. Try seraching for his post.
If you find your visas are running out when you get near Vientianne, you can extend, but I reckon it is better to go to Thailand, then cycle that side of the Mekong. More to see, better towns, better food, etc. You can cross back into Lao at Mukdahan/Savanakhet Bridge (must take a bus!) or carry on to land crossing near Pakse. If you go the Thai way you miss the Khong Lor cave, but that is about all. You get 15 days free entry into Thailand if you have no visa.
In Southern Lao, don't miss the Bolaven Plateau, go all the way out to Attapeu (can bus back if you don't want to repeat, then loop back via Tad Lo for some very refreshing waterfalls and cheap riverside bungalows.
Hope this helps, sorry about the previous time wasting. There may be a few minor errors as I am doing it from memory, but with a map you should be able to work it out. The GT Riders map is best, not easy to get it in Lao, Luang Nam Tha is probably your best bet.
Another thought just in case you change your plans, do you know that you need a visa for Vietnam -they are not available at the border. Also it is time critical - you have to state entry date and if you enter later you lose that time. Sorry if you already knew this.
Do come back and cycle Vietnam on another trip - you can get 3 month visa and there is so much to see and do.
Whatever, enjoy!
