why is vinyl chloride is nonreactive to nucleophilic substitution reactions?

Nucleophilic substitution via the SN1 or SN2 mechanism does not occur with vinyl chloride (or other vinyl halides or aryl halides) as it has an unsaturated carbon centre. So the bond between the chlorine and the carbon in the double bond is much too strong - stronger than that of an alkyl chloride - to be broken by a nucleophile (SN2). Also the electrons of the double bond repel the approach of a nucleophile.
Vinyl chloride does not undergo an SN1 reaction either because the carbocation intermediate it would produce would be far too unstable and so is not readily formed.