is it true that vinyl and phenyl carbon never undergo substitution reaction niether by sn1 nor by sn2 or both USUALLY don't undergo substitution

Vinyl and phenyl carbons are bonded with double bonds. So it's very very difficult for a nucleophile to start the bond cleavage. But I'd still go for "usually" wala option because double bond can be cleaved under certain cases like we observe the nuceophilic substitution in halobenzene in presence of NaNH2 by benzyne mechanism. In that case too, the partial double bond character is effectively cleaved by the base NaNH2.