Two effects make the carbon–halogen bonds of vinylic halides(double bond at alpha position) shorter and stronger: (1) The carbon of either type of halide is sp2 hybridized, and therefore the electrons of the carbon orbital are closer to the nucleus than those of an sp3 -hybridized carbon. (2) Resonance of double bond with lone pair in halogen atom makes carbon–halogen bond stronger by giving it double-bond character. 

Vinylic halides are unreactive in SN1 reactions because alkenyl cations are relatively unstable and do not form readily. They are unreactive in SN2 reactions because the carbon–halogen bond of an alkenyl halide is stronger than that of an alkyl halide, and the electrons of the double bond repel the approach of a nucleophile from the back side.

Source: Solomon's organic chemistry