I have been on the Vientiane-Savannakhet bus half a dozen times. Most of the buses actually go to Seno, which is on Route 13, and then continue to Pakse. Savannakhet is 30km west of Route13, but there are always many songthaew waiting at the Seno bus station to take you onto Savannakhet (45 mins).
There are a couple of buses that go straight to Savannakhet but they are the minority, and one of them is the nightbus which leaves Vientiane southern bus station about 8-9pm. Another disadvantage of the nightbus, in addition to the info in posts above, is that they drop you at Savannakhet at 4.30 to 5am when everyone is still asleep, so you have to wander round the deserted streets for 2 hours till the first noodle shop opens.
Most of the buses stop in Thakhek bus station for 30 mins for a meal. The toilet at Thakhek bus station is the worst I have ever smelt, a year in India included, and you have to pay for the privilege of burning the hairs from your nostrils with the ammonia also.
So yes, I would take an early morning bus. If you are lucky it can take as little as 8 hours. The longest I ever took was 14 hours but that was over Pimai holiday.
Edited by: nabanga