I stayed in the Nongsoda Guesthouse on 8th and 9th! But what a scramble at the two immigration offices on the border. The bus I was on was full to bursting. I didn't think much of Savannakhet; an overgrown village, another traveller said to me. Not a single decent restaurant with a view of the river. But the beer is good (and the wine, people say, but I wouldn't want to drink a whole bottle with a meal).
If you don't want to go in the middle of the night, take a bus to Roi-Et at about 9.30. Change at Payak (Payakkhapum Phisai), or if you miss the change (I did), you can go on to Roi-Et and catch a bus at 12.30 for Mukdahan. I am not sure if you can go straight on to Savan; if not, go to the Huanam Hotel in Muk, about B.300; very basic, but clean and friendly.