Rashi Breishis 23:1 says that when Sarah was 20 she was beautiful as when she was 7. Mishlei 31:30 says שקר החן והבל היפי אשה יראת ה' היא תתהלל. Why is Rashi saying that Sarah was beautiful if it is vain?
The Pele Yoetz answers this question by saying that if one can get a beautiful wife and she is also fearing of God then that is the best combination. I think he says the reason is that a pretty wife will help her husband from sinning.
The rule of שקר החן is for someone who cannot find any God fearing wife except a non-pretty one, in that case we say שקר החן.