Definitely sounds like a vacuum leak to me, especially if your hear the vacuum pump running several times after start-up. Best way to find the leak is to remove the glove box and right under dash panel to gain access to the vacuum manifold for ACC vent controls. Carefully disconnect and test all lines for leaks with a vacuum tester. ACC vacuum line schematic will be needed to know where leaking lione goes if you find one, but it sounds like it is probably the footwell flap vacuum element. These elements can be a bear to replace, but not impossible. Good luck.