The note doesn't say anything about letting Don know about the special request itself. Just the results of it. So Vinny could still tip Don off, if he felt so inclined. I don't know if Vinny is suspicious of Caesar though, or would find the request suspicious.
Good call. That would be interesting. "Don, I have some bad news. It appears that Ceaser is planning on conquering FAQ." Of course the more probable course of action is that he'll try and use Bunny and that will tip Bunny off. Then one suggestion later Don will say "He's probably just being proactive! He does that sometimes. I'm sure we have nothing to worry about.