Hi Snowflake,
You have 3 ways to go Vientiane from Siemreap:
– Fly: it’s the fastest way but not cheap at all comparing to other choices
– Work your way down around to the Laos/Cambodia border and then up Laos into Vientiane. You can do it in 2 days  but I would advise taking at least 3 if not a week or more to stop at some of the sights along the way. The buses in Laos and Cambodia are not all that comfortable so I advise against this route if you are in a hurry.
– Take a bus or fly to Bangkok and then take night train up to Nong Khai and then minibus/bus to the border and then into Vientiane. If you want to save money I believe this would be your most comfortable way to do this trip.