Just to answer my own query, Tango SIMs, widely available, have decent coverage in Vientiane, Luang Prabang, and points in between such as Vang Vieng and even in the mountains along Highway 13.

500 Thai Baht for the SIM and not much airtime. 50,000Kip for a top up. Don't expect a lot of talk credit for a call to the US. It seemed about $1.50-$2/minute.

Also, don't expect to be able to use a Thai SIM in central Vientiane, though Thailand is close enough to hit it with a ball of sticky rice thrown across the Mekong from a Vientiane riverside restaurant. Good coverage by the time you get close to the Friendship Bridge, however.
Rich