Many Paris travel agencies stayed open until midnight to deal with a last-minute rush of panicked would-be vacationers. They were trying to beat the Socialist government's strict spending limit on foreign travel, which came into force Tuesday. The measure, part of an economic austerity package, limits French tourists to $270 each in foreign currency for the rest of the year. ''Vacations are sacred,'' one Frenchman said. ''They can tamper with our taxes, OK, but not with our vacations.''