Why is the dipole moment of ethyl chloride larger than that of vinyl chloride?
I see two explanations for why the dipole in vinyl chloride is smaller than ethyl chloride.


Consider a resonance structure where the chlorine donates electrons to the alkene. This puts a formal positive charge on chlorine: 



This would be a very minor contributor given the electronegativities and the mismatch in orbital size. However, this is not possible in ethyl chloride at all.
The more likely explanation is that an sp2-hybridized carbon is more electronegative than an sp3-hybridized carbon. Since the electronegativity of chlorine will be closer to the electronegativity of the carbons in vinyl chloride than the carbons of ethyl chloride, the C–Cl bond will be less polarized in vinyl chloride, giving a smaller dipole.