To deal directly with the original posters circumstances. There is no problem. Brazil requires vaccination if you have been in a Yellow Fever area within the last 6 days or 10 days or 2 weeks or something (cant recall exactly but I'm quite certain it IS NOT 90 days). A trip in September is definitely not an issue to Brazil as far as Yellow fever vaccinations are concerned.

All suggestions to get a new passport or modify a passport to evade a vaccination requirement are just plain silly. If it were an issue (which it isn't) I'd still be highly confident they probably would not catch it. I have never been ask to prove vaccination anywhere except Cambodia, which doesn't even have any vaccination requirement, and that was only in an attempted shakedown for a "tip".