hi Matthias,
if you configure MAB, so you're using NAP right?
in authentication section you can list the MAC addresses and on right coloum there is user group to associate the MAC with group.
and in every group you can define the VLAN id.
but btw is catalyst 2950 support MAB, i ever try but there is no command for MAB (dot1x mac-auth-bypass).
br,
aw