Vientiane

Given your stated interests, I think you could easily spend a few days in Vientiane. The tourist center is a stretch along the river, quite a nice location, and a good place to spend some hours in the little cafes and bars along the banks. There's a French restaurant in that section, name escapes me, but you should be able to find it, in all the books, that did an excellent meal, much better than anything similar that I found in Vietnam or Cambodia.

You need to visit Vientiane if for the French Colonial Style Houses alone. A couple of days is about enough, unless you start to venture outside of Vientiane...say to Nam Ngum Dam or Thalaat maybe. The sights to see are the Annusavali or " Patuxai", a massive monument in the center of town, it has a fountain that does a small light and music show in the evenings. Also there is That Luang, a large Golden Stupa that is considered to be the National Monument, A little outside of Vientiane is the Buddha Park, it is very interesting, it doesn't just contain Buddha statues but also has a mixture if Hindu and FolkLore Figures.
If you have time for a side trip the Vientiane Zoo is a nice place to visit, but it is quite a ways outside of Vientiane in Ban Ghurn. As far as good eats go traditional dishes can be found in Riverside Cafes along the Mekong. There are a lot of reasonable guest houses and hotels. One of my favorites is the Mali Namphu near the Fountain (Nam Phu).

I would also recommend spending a few days there but if you are disappointed dont' be surprised. To get the most out of your stay try and coincide with a festival in October...

JB2

I think it's the Cote d Azur restaurant of which you may be thinking. They also do good pizzas.

La Silapa does superb French food but only if you've got an awful lot of cash burning a hole in your pocket.

La Vendome behind Wat Inpeng is a good and much more financially friendly alternative for cheapskates like myself.

There's also another one called La Terrace on one of the sois running back from the river.

Basically they're everywhere. Can hardly walk down the street these days without tripping over a French Restaurateur it would seem.

And the Scandinavian Bakery - good baguette sandwiches - is a favorite of westerners.

Vientiane then HCMC. I wish you could visit HCMC first then you will appreciate the quiet and peaceful atmosphere of Vientiane, which is far different from the noise of millions motorbikes in HCMC.

check out the arobics Lao style on the river! Well worth a look if you can see thru the dust of the main road construction!

Certainly agree that Vientiane is worth a few days.

Also worth exploring is the Morning Market and the Wats near the Mekong especially Wat Sisaket with all its small buddhas.

To get to the Buddha Park catch the #14 bus from the Morning Market. Much of the ride is along the Mekong with views into Thailand. The bus also goes past the Friendship Bridge.

Many good suggestions above. The fountain area is also good area to star for first time traveler to ViangChan.