I second Porte de Vanves...I never found anything beyond the people selling Eiffel tower trinkets and crap at Clignancourt. Try as I might I never found these mythic vintage stores there, plus the metro there was soo long. Vanves is much more pleasant and better finds.
A funny story, one night on the metro I saw a couple on the other side completely in vintage style; his was this matching hunting tweed outfit, with britches, knee high socks and spaniel!. And his girlfriend was in a 50s style vintage outfit. I so wanted to take a pic of them, and spent the metro home thinking where in the world he got that matching tweed hunting outfit.............A few months later, I go to Porte de Vanves flea market and the same guy is there talking about vintage with a vendor and he's wearing another fabulous vintage outfit (britches again). If its good enough for him, its good enough for me.