Why is vinyl chloride is hydrolysed more slowly than ethyl chloride?

Vinyl chloride is hydrolyzed more slowly than ethyl chloride because the ##"C-Cl"## bond in vinyl chloride has some double bond character.

There are two reasons for the slow hydrolysis of vinyl chloride.

(a) The carbon atom in vinyl chloride is ##"sp"^2## hybridized.

The ##"C-Cl"## bond contains more s character from the ##"C"## atom, so the bond is "tighter" and more difficult to break.

(b) The ##"C-Cl"## bond in vinyl chloride has some double bond character.

(from www.askiitians.com)

Vinyl chloride is a hybrid of two resonance structures.

The second structure is a minor contributor but, to the extent that it occurs, it gives some double bond character to the ##"C-Cl"## bond.

In ethyl chloride the ##"C-Cl"## bond is a pure single bond.

Thus, vinyl chloride undergoes hydrolysis more slowly than ethyl chloride.