You will do most things in Lao kip but it's easy to change USD and THB in any town big enough to have a decent sized market.
The exchange rate for Thai baht at the end of 2014 was between 246 and 250. For USD it was a bit over 8,000. I never felt ripped off and a few times felt the money changers could not be making any profit from my transaction at all.
It's much much easier than changing money in Thailand, which is a total pain. I never bothered trying to change money at a bank in Laos. If you want the best exchange rate it's always worth finding the money changers near the market, they will always have better rates than exchange booths in the tourist area, if there is one. Make sure you don't leave it too late. Money changers close in the afternoon before dusk except a couple in tourist areas such as by the fountain in Vientiane, where the rates may or may not be competitive.
A bonus tip for Aussies, most ATMs only let you withdraw 1,000,000 or maybe 1,500,000 kip per transaction and charge 20,000 kip. That can suck since Aussie banks will also charge you about $5 plus a percentage. ANZ ATMs allow you to withdraw 2,000,000 kip but charge 40,000 kip - the good news is that if you use an ANZ card they don't charge the fee!
EDIT The one time you will have to pay in THB or USD is if you want a visa on arrival, this was $30 for me five or six weeks ago and I believe there was also a $1 or $2 fee for something or other. The price in THB is higher than the price in USD so it's worth bringing dollars.