i know that many Rabbanim from Flatbush do take vacations during the summer yet in almost every case there is a sign up with the Rav’s vacation location and/or summer phone number or a listing of Rabbanim who can be called if there are shailos. There are also many Rabbanim who are home who are available to answer shailos.
The Rav of my shul does not take a “vacation” but does go away for a shabbos or 2 to visit his children. Many years ago I had a shailoh during the nine days, called my Rav and on his answering machine were the names and telephone numbers of 4 poskim to cal if there was an urgent need for a psak.
I have heard that R’ Asher Zimmerman TZATZAL, did not go away for the summer so he can be home to answer shailos.