Most tour operators in Vang Vieng offer kayaking by the half-day or full day. Full-day trips typically include a stop at Tham Chang cave or one of the blue lagoons along the way, and end with a tuk-tuk ride back to town. You can rent kayaks independently if you prefer your own pace, but a guided trip adds context and handles logistics.
The best months are November through February, when the water is lower and clearer. During the monsoon the river runs fast and brown, and some sections become difficult. Combine kayaking with tubing on the same day if you want a more varied experience; the tubing section is mellower and good for a lazy afternoon after the active paddling.