Hmm Pl. Thank you for the clarification! Still a little confused but that helped.Don't get confused, so far there is only one confirmed case. The 500 people exposed is estimated and exposure does not separate those vaccinated from those unvaccinated. This quote from the OP will shed some light for you:
So of the 500 people unvaccinated, they're monitoring about 40. Which means the 460 others are likely vaccinated or otherwise immune. This isn't an outbreak (yet).
EDIT:
For others, 500 people did not catch measles. 500 people were EXPOSED to it, they're monitoring 40 of those. This is not a measles outbreak.