In this letter, the polarimetric synthetic aperture radar (SAR) interferometry (PolInSAR) statistical model is studied using the alpha-stable distribution. Based on this model, a vector coherence formulation using fractional lower order statistics is proposed. This vector coherence is the generalization of the vector coherence in the paper of Cloude and Papathanassiou when the PolInSAR data are non-Gaussian and conform to the symmetrical form of the alpha-stable distribution. The standard coherence optimization method in the aforementioned paper is modified based on this generalized vector coherence. Results were demonstrated using a small L-band PolInSAR data set and suggest that this proposed coherence optimization method reduces artifacts in the optimized phases in certain areas.