Hi Michael and Ziners.
Not sure I can be of much help as I am in Aus and my notes on Laos are in Toronto.
With regard to booking guesthouses in Laos, we found it difficult. We would phone ahead and be told "all rooms full", and we came to realize that they would rather say that and leave the opportunity open for someone to walk in than make a booking and have nobody show up. It is true that in the busy season if you are arriving late in the afternoon you are likely to have to do some shopping around for a room but there seem to be enough and May is not high season for Laos
In Vientianne you need to be in the area by the fountain. Nice mid range guesthouses and plenty of good restaurant and bar options. Close enough to the river to stroll down there, close to the market if that interests you and plenty of transportation. The French connection is alive and well in Vientianne.
If you are flying Air Bangkok to LP and from there to Vientianne and back to Bangkok then it would make sense to buy the Air Bangkok Pass for the 3 flight minimum. Last year your international flights (Bangkok - Laos) would cost $80 US and the internal Lao flight $50 US. It is a great deal and some people find it can pay to buy 3 flights and only use 2 of them. As with all passes there are rules and regs to be followed but we found it a bargain. I don't think your travel agent can book this for you but you can do it on the internet.
Sorry I can't remember the exact names of our guesthouses and an approximation in Laos just won't help, lots of places sound very much the same.
Bangkok immigration and customs is not a big deal, you will soon be through and on your way. By the way Bangkok Airways have a great lounge at the airport in Bangkok, free beverages (none alcoholic) sandwiches, pastries and best of all free Internet. So if you have some spare time between flights, head over there.
Sorry I can't be of much help with your specific questions but hope this adds to the picture.
Enjoy your trip.
Regards Sue, Waterloo ON currently on the road in Australia.