I've been doing some reading in preparation for our trip, and I'd like to mention three books that I found particularly engaging as well as historically and culturally instructive.
1. The Latehomecomer (2008) by Kao Kalia Yang. Memoir of a Hmong woman whose family escaped from Laos.
2. In the Shadow of the Banyan (2012) by Vaddey Ratner. Listed as a novel, but actually a memoir of a woman whose family escaped from Cambodia.
3. The River's Tale (2002) by Edward A Gagan. An earlier travel memoir by a reporter who traveled the length of the Mekong (north to south). Relevant sections on Laos and Cambodia are pp. 115-267. (South Vietnam, pp. 268-313)
I've also enjoyed the PBS series The Mekong River with Sue Perkins. Part 2 includes Cambodia and Part 3 includes Laos. Of the PBS series on Vietnam, I found episodes 1 and 2 (1868-1963) the most instructive.
Looking forward to the trip.