I haven't tried it personally, but know people who swear by it - put the Vick's on the bottom of their FEET and then put on a pair of heavy wool socks. Really works to ease the coughing and let them sleep.
My grandmother was from the old country and I am so glad I took her advice when I had children of my own. You put mentholatum rub (has to be metholatum not Vick's) on their chest, back of knees and then the bottom of their feet and then put a pair of heavy cotton socks on the feet. Every time I used this they would sleep through the night and wake up feeling much better.