No vaccine is 100% effective, so she could still potentially get a disease if she was around a dog who had it even if she was vaccinated for it.
For me, it would depend on what and when the dog was vaccinated.
If a dog had none at all, I probably wouldn't let Ellie hang around.
If they had puppy shots and one year booster, I personally wouldn't be too concerned. Then again, whatever I felt like doing at the time is what I'd do.
One of the reasons I won't Ellie go to dog parks is because you really don't know what diseases those dogs are carrying, so I am somewhat cautious.