The quasielastic barrier distribution of 51V+248Cm was extracted by measuring the excitation function of quasielastic backscattering using a gas-filled recoil ion separator, GARIS-III. The obtained barrier distribution is well explained by the coupled-channels calculation, indicating a significant effect of the rotational excitation of deformed 248Cm. From the measured average Coulomb barrier height and deformation parameters of 248Cm, the side-collision energy leading to a compact configuration of colliding nuclei was obtained. The relation between the side collision energy and the excitation function of the evaporation-residue cross sections in the 48Ca+248Cm system was evaluated as a reference for the 51V+248Cm case. The optimal reaction energy to synthesize a new element 119 at the 51V+248Cm fusion reaction (3n and 4n channels) was estimated with an aid of these experimental data.