Is it wise to use mathematical morphology and Gabor filter to extract blood vessels in the same algorithm?
In a word, yes. 

Mathematical morphology and harmonic analysis (Gabor for instance) are two families of tools, relying on different sets of assumptions, pertaining to image formation models. Harmonic analysis dwells in vector spaces, addressing better linear or superposition  effects. Mathematical morphology, built on lattices, could be more adapted to connected objects, discretized levels, non-linear effects. Both types of contents coexist in images, so mathematical morphology and  harmonic analysis can be complementary tools for complicated tasks. 

Some tools reside at the intersection, like morphological wavelets, some links relate linear filters and structuring elements (logarithmically, through the Cramer transform, see fro instance An Explanation for the Logarithmic Connection between Linear and Morphological System Theory). However, it remains wise to use the right set of tools when it is at its best.

For instance, in the most simple signal processing applications, where information is degraded by both  impulse and Gaussian  noise, it is common to first use a non-linear median filter, and a linear low-pass one.

Finally, let me mention a recent thesis related to your topic: RORPO : A morphological framework for curvilinear structure analysis; Application to the filtering and segmentation of blood vessels, where mathematical morphology and harmonic analysis are discussed.