Reconfigurable intelligent surface (RIS) can provide stable connectivity for vehicular communications when direct transmission becomes significantly weaker with dynamic channel conditions between an access point and a moving vehicle. In this paper, we analyze the performance of a RIS-assisted vehicular network by coherently combining received signals reflected by RIS elements and direct transmissions from the source terminal over double generalized Gamma (dGG) fading channels. We present analytical expressions on the outage probability and average bit-error rate (BER) performance of the considered system by deriving exact density and distribution functions for the end-to-end signal-to-noise ratio (SNR) resulted from the finite sum of the direct link and product of channel coefficients each distributed according to the dGG. We also develop asymptotic analysis on the outage probability and average BER to derive diversity order for a better insight into the system performance at high SNR. We validate the derived analytical expressions through numerical and simulation results and demonstrate scaling of the system performance with RIS elements and a comparison to the conventional relaying techniques and direct transmissions considering various practically relevant scenarios.