Greenbug, here is what I know. Most of this info is about 6 months old, but should be valid.
Tamba to the Malian border is perfect tarred road, although itmay have some very minor potholes by now.
Mali border to Kayes is very badly potholed, but as long as you keep going slowly easily do-able in any kind of car. I have heard that hey are working on tarring the road, unconfirmed though.
From Kayes you have various options, but easiest is to head east, via the old french fort and the small powerstaion, all along the river to the town of Diamou. Here you cross the railway lines and keep going on a tarred road which appears out of nowhere. You pass an old cement factory and immediatley afterwards you turn right and follow one of the tracks. They all follow the river and after about 13 kilometres on bad 2 track road youget to some very very beatiful water falsl with no villages nearby, perfect camping spot.
Next day DON'T continue along this road. It will be very difficult for a VW, rather turn around, go back to teh tarred road, turn right when you reach it, keep going, folowing teh tarred road for about 30 kilometres, then pass throuhg a town called Selinkegne, then head on until you reach the other side of the river at Bafoulabe. This is where the Bafing and Bakoy rivers merge to from the Senegal. Take the ferry across the river and head for Mahina, then Manantali, then Kita, then Bamako, all plain sailing.