Multiple id problem

Hello. I have an instance that I'd like some input-suggestions on if anyone
would be so kind. We have a vendor table with each vendor ID being unique.
One problem is, some vendor numbers have changed so there is a current # and
an old #. Because of the multiple vendor #s the primary key is an auto #. The
main dilemma I'm having with the design is that at least one vendor (maybe a
few more but I'm not sure yet) has two old numbers and one new/current #.
There are hundreds upon hundreds of vendors, and I'm wondering if a third
vendor # field is the way to go. It seems like a cumbersome solution,
especially since it's (apparently) only one vendor. On the other hand,
although it's unlikely, the situation might come up again and I want to be
ready for it even if it's 5 years down the road.

I'm grateful for any ideas.