Vietnam forum

Posted from within Vietnam.

Don't even think of using a travel agency, especially not a Hanoi based fake Sinhcafe :-p

If you speak to the riders direct you'll be able to get a clear idea of who you're riding with and will probably end up paying less.

Look out for the guys in Nha Trang with good motorbikes; usually with badges saying "Easy Rider" or "Bike Tours"). Chances are they'll find you if you're walking around in the daytime anyway. Have a chat with them about the route they take, places they stop off at and look at their book of photos and testimonials.
Feel free to take their card and walk away if you find them too pushy or difficult to understand: there are plenty of riders to choose from and the best ones are very good guides. If you have any worries about the longer ride get a day trip around the Nha Trang area first so you can judge their driving and demeanour.

The blue-uniformed original Easy Rider group - older guys based in Dalat - can be quite disparaging about the Nha Trang Easy Riders ("Not educated" said mine, managing to sound even more French than usual) but then they would say that about the competition. The "originals" tend to be older and enforce certain standards such as not drinking alcohol in the evenings which you may or may not prefer. You won't find them touting for business in Nha Trang but if you're determined to use their services you can probably prearrange for someone to meet you there and ride back to Dalat via their website.
http://dalat-easyrider.com/

Everyone else will probably tell you they're an "original" Easy Rider anyway, and many of them will have been doing bike tours for at least as long. It's best to focus on whether you like the original rider.

More good info in this thread
http://www.travelfish.org/board/post/vietnam/6282_the-real-easyriders-
(mod: please could you make that thread a sticky)