I don't know about straight vinegar, but a common solution for clearing out clogged drains is to dump half a cup of baking soda down the drain, followed by half a cup of vinegar and then a whole lot of boiling water. I'm not sure if the reaction with the baking soda changes the way the vinegar would affect the pipes, but if you pour some baking soda down before your foot soak, and you'll have a very clean drain! =)