Can't speak to syncing issue, but VS25 is currently J6, so it looks like there should be just enough seats. The more convenient VS137 with 12:45PM departure is completely zeroed out in J. Unfortunately, LHR-JFK frequencies are pretty limited from now through March 26th with only 3x VS and 1x DL flights on many days and there are only 2 of those flight options that work for TLV-LHR connections (both VS flights). Daily frequencies increase after that.