Using operator-valued \(\dot{C}^{\alpha }\) -Fourier multiplier results on Hölder continuous function spaces with values in Banach spaces and the Carleman transform methods, we completely characterize the \(C^{\alpha }\) -well-posedness of the vector-valued first order degenerate differential equations: \(\mu * (Mu)'(t) + \beta * u(t) - \gamma *(Au)(t)= f(t)\) on \(\mathbb {R}\) , where A, M are two closed linear operators in a complex Banach space X such that \(D(A)\subset D(M)\) , \(0<\alpha <1\) and \(\mu ,\ \beta ,\ \gamma \) are complex Borel measures on \(\mathbb {R}_+\) .