Just returned from 5 1/2 weeks traveling around Cambodia and Laos and figured I'd offer some thoughts and suggestions. All in all, we found most of the Cambodia food a little blah (you really had to go to town with the condiments on the table in order to create better depth of flavors) and generally loved Laotion cuisine.
LYLY -- on the main road leading from the tourist spots (Pub Street & The Alley) up to the airport road, right next to the mall with the grocery store on the first floor - Looks like your typical random meat spot, but there are a few surprises: yummy pressed tofu and the ability to add it to some stir-fried veggies for a good veg meal, an amazing noodle soup with beef bone, and if you're lucky, good old fashioned rice porridge with some great mix ins. Highly recommended - we went back for a 2nd time!
Avoid KIMLY (near the crab market) at all costs. An inability to be friendly and at all flexible is the rule here. Also, my stir-fried veggies came swimming in oil. We're talking a POOL. Ugh.
CURRY HOUSE - An amazing tiny gem of a place, right on the street with most guesthouses (across from Blissful). A nicely thick and yummy lentil dal (made with green lentils), well cooked vegetables, nice chapatis and a really friendly and accomodating owner. A nice respite from stir-fries...
SALAPA FISHERMAN'S HAVEN - a guesthouse one hour north of Vientiane on the lake formed by the Nam Ngum Dam. The owners are Laos, but the husband spent most of his life in France. His wife (& their staff) create miracles in the kitchen, seriously. We arrived and just told him that we loved Lao food, lots of spice and lots of variety and to just feed us their specialties. Over the course of three days, we had various grilled and steamed fish with veg accompaniments, multiple flavors of chili sauces, noodle soups, fish & coconut milk & ginger steamed in banana leaves, amazing, amazing local fruits and other things. Not cheap, but worth it from the perspective of really knowing what Lao food can be when a little love and care is put into it.
KOREAN - There are two Korean places on Rue Hengboun, off Rue Chao Anou, beside the big grocery store/home store (I think it's called Ideal Home). The other further away from the grocery store is definitely worth a visit. It was filled with visiting Korean businessmen and the food was a step above your average Korean. The soybean paste soup was filled with tofu and very miso-ey and the normal accompaniments were a mixture of veg & non-veg, with some seriously stinky kimchi. Yum!
In general, that bit of Rue Hengboun between the grocery store heading out of the center toward Khun Bu Lom Road has a bunch of great food stalls and vendors: noodle soup, rice porridge, grilled meats, duck, and other various fun things. A few meals could easily be consumed here...
We also visited Ban Na Hin, Tha Kaek, Pakse, Bolaven Plateau and Champasak and had a few good and a few not-so-good meals. Nothing more really worth really mentioning.
My last recommendation is to get out of the cities and into the countryside, either by doing a kayaking or trekking tour, and stay in the villages. The food cooked at homes that we stayed in was universally good, and people were THRILLED that we were willing to taste different things and that we loved the spices, flavors and textures. Plus, eating with your hands using sticky rice is a blast!