QUESTION:
The vet said he "believes" she has distemper by her symptoms but was not 100% positive. She only had 1 shot when she was a baby and she is almost 3 years old now, so most likely that is what he has. The healthy ferret is almost 1 year old, he had a shot when he was a baby as well, and I just got him vaccinated again a week ago. What would the chances be of him catching it if I let them near each other again?

ANSWER:
What are some of the signs your ferret has that the vet suspects has distemper?

Distemper is an airborne virus and lasts on you and your clothing for at least 8 hours, so if you have been handling both ferrets without washing and changing your clothes etc.. then the other ferret has most likely been infected already.

He should have had 3 distemper shots 14 days apart, starting at 8-9 weeks of age to be completely immunized, so there is still a chance that he could come down with it also, that is if your other ferret really has distemper!