If it was the Merial Purevax there are statements by vet after vet
stating that they are seeing about 1/10th to 1/20th the number of
reactions as they saw before with early vax.  Of course, that still means
that they are seeing reactions.  It's good that the reaction was mild
enough that it was only vomiting rather than collapse from blood pressure
dropping, blood fluid loss, and tissue inflammation, but some ferrets
just are individuals who don't cope with some vaccines.  In 23 years
we've had something like 5 ferrets who could not tolerate either CD vax,
or rabies vax as well as CD vax.  We haven't have any personally who were
only allergic to rabies vax but certainly have read of ones others have.
The reactions can be scary to observe, can't they?
 
My personal take on it, having had such reactions myself as well, is that
I find anaphylactic reactions harder to observe in someone I love than I
find them to go through.  Of course, without prompt treatment I'd sing a
different tune, so given my terrible singing voice it is good that I've
never gotten to the point of needing a stoma or respirator for one or
you'd al be holding ears.
[Posted in FML issue 4543]