What evidence do you have that productivity is elastic in that way? I'm particularly curious how "far out" the effect you're describing goes: is it still elastic enough to cover an extra 3 weeks of vacation per year?
Companies would squeeze more work from their labor pools if the government mandated longer vacations, especially from homogenous/low-wage workers due to the mismatch of labor supply and demand that points in the firms' favor at the moment. I don't think people would necessarily feel motivated to work harder for the rest of the year after a long vacation, but I think many would feel like their job security threatened by someone who was willing to put in more hours than them to make up for this new government policy.