I think that if you attempt to follow every compliment with a semi-anti-complement that you're zeroing in their focus on praise and compliments rather than teaching them "it doesn't matter".
It serves your kids better to teach them how to graciously accept compliments, and how to compliment others (in other words, how to see lovely things in other people).
Kids in general, developmentally, are pretty egotistical (both in a "well, of course I'm awesome" sense but also in a "Wow, that bad thing happened, it must be *all my fault*.). I think very few creatures have the self-esteem of a primary-grader (6-10y) (of course, there are exceptions!) So if your daughters are "vain" it's probably NOT because they get complimented by people--they may just feel competent and good because they are secure and haven't had people telling them otherwise (thank god).
Beauty isn't skin deep. There's nothing wrong (or essential) about physical beauty. Bodies/Faces change over time (beautiful kids don't always retain their beauty in adolescence and beyond, and kids not thought of as beautiful sometimes fit the societal profile of it as they mature!), and so do other things that are complimented--athletic ability, musical ability, intelligence...all of these things can be relative as the person gets older/moves in different circles. I think you need to be really careful about not unintentionally hyperfocusing your kids on physicality (which can happen if you are so upset about positive references that you want to knock it down!) or setting up a situation where your kids think that mom thinks they're bad (which if you find yourself needing to correct the nice things people say about them, they may get that feeling.)
How are you at accepting compliments? Do you model it well? Or are you automatically rejecting or overly dependent? As hard as it's been for me to do this, I think I've done a good job teaching my kids how to accept compliments graciously and to enjoy complimenting others. My kids have had their egotistical/I'm awesome undeserved moments, but they've moved in and out of it with their peers by my observation. Now that she's entered into puberty, I notice my daughter becoming more inwardly critical (even though she's still complimented, and doesn't have a lot of exposure to mainstream media stuff), along with some of her friends who are going through the same. Getting told she was beautiful didn't seem to help or hinder that. She often thinks that she has poor skills in things that she's always excelled at that don't have anything to do with physical appearance at the same time too.
So I would relax about it if I were you. Or talk about it, acknowledging that she enjoys receiving compliments from others, does it make her feel good? How could she "pass on" that feeling to other people (not just appearance)?