The interface between the PIX and the 2950 will be configured as a trunk port on the 2950. Trunk ports by default will carry all VLAN traffic. On the other switch ports on the 2950 just assign them to specific VLAN's for your vendors or dmz.
On the PIX, all you should need to do is something like the following:
interface ethernet2 vlan10 physical
interface ethernet2 vlan20 logical
...
nameif vlan10 dmz security50
nameif vlan20 vendors security30
...
ip address dmz x.x.x.x netmask .....
ip address vendors y.y.y.y netmask .....
So the dmz physical NIC (ethernet2) is now basically two interfaces, one called "dmz" which maps to the vlan10 ports you have configured on the 2950. The other is called "vendors" and it maps to all the vlan20 ports you have on the 2950. Traffic between vlan10 and vlan20 will be routed to the PIX and back out. For this reason make sure the appropriate PIX interface IP address is the default gateway on all the hosts.
This is a major change to the config and will cause outages, so you'll have to plan accordingly. Since you're doing failover you'll have to assign a failover IP address to any new interface you create, logical or physical. Keep in mind your "failover link" or "failover lan interface" cannot be a logical interface. I would suggest you organise an outage, turn off failover for the duration of the change, make all the necessary changes on the primary, ensure it's all working as expected, then put the failover unit back on line.
