The main objective of this work is to analyze finely the influence of cross-linking on the mechanical properties of PBT (Polybutylene Terephtalate)-based vitrimers. Tensile tests were carried out at four temperatures (80°C, 100°C, 130°C and 160°C) on a series of vitrimer specimens made by injection molding for which the cross-linker concentrations were changed by small increments of 0.25%, from 0% to 2%. The displacement and strain fields were measured on the specimen surfaces through 3 D DIC (Digital Image Correlation) in order to analyse the strain localization/delocalization phenomena that occur during the successive stages of the deformation process. In particular, we measured the yield strain Y  (onset of strain localization), and the strain at neck stabilization NS  (beginning of the strain delocalization phase). When the degree of cross-linking increases, we observed two complementary effects leading to the decrease of the strain range during which plastic instability develops: Firstly Y  increases. Secondly, and this is the main cause of the plastic instability strain range reduction, NS  decreases. This latter effect results from the limitation in extensibility of the macromolecular network in the solid state caused by cross-linking. More specifically, cross-linking leads to two distinct modifications of the macromolecular network that both contribute to the reduction of its extensibility. The first is the decrease of the chain length between the network nodes. The second is the preorientation of the macromolecular network that occurs during injection molding in the case of vitrimers with high viscosities due to cross-linking. Eventually, when Y NS  → , the suppression of the yield point on the tensile curve was observed for the most cross-linked vitrimers. Furthermore, in the temperature range of investigation, NS  was found to be independent of temperature.