The vet is necessary in the sense that they will confirm if its YF with a scrape. If it is yellow fungus it's very difficult to get rid of. From what I've seen, the lamisil just kind of keeps it at bay as opposed to getting rid of it completely.
Although, I dont have personal experience with YF as I've never had to treat a dragon with it.
I would guess at this point if the YF is the cause of the toes falling off, its systematic and most likely won't be going away. But like I said, no personal experience with it so hopefully someone with some will chime in on a prognosis.
That's why it's important to get to a vet. They can confirm it or not. From there you'll know what's necessary.
-Brandon