The formation of the first aromatic ring plays an important role in defining the chemical reaction pathways responsible for polycyclic aromatic hydrocarbon (PAH) and soot formation. In this work, the relative importance of cyclo-C-5 species formation with respect to the cyclo-C-6 one from the attack of vinyl radical on 1,3-butadiene has been investigated through quantum Rice-Ramsperger-Kassel (QRRK) theory and quantum chemistry. A fast and accurate method, which differs from the standard G2MP2 method in that geometries were optimized with B3LYP/6-31G(d,p), has also been proposed and validated. Kinetic constants for each elementary process involved in the reaction mechanism were determined with conventional transition-state theory. It has been found that the rate of formation of C5H6c5 is always larger than that of C6H8c6 in the whole temperature range investigated (that is, 500-2000 K). All of the results presented in this work lead to the conclusion that the reaction paths involving cyclo-C-5 species cannot be neglected in the detailed kinetic modeling of combustion processes when the first aromatic ring formation is involved.