There is another way but I would not call that an easy way - part single trail and it might be muddy and you will certainly not arrive in Vientiane before 1 o'clock.
Another option is to drive from Paksan to Pak Ngum and a bit past the bridge over the Nam Ngum river, head south on dirt roads to the Mekong and follow the Mekong till you reach Vientiane. Not sure about the condition of the road but part may still be quite bad although the last time I was there they were upgrading at least part of the dirt road.