Vitrimers are polymer networks with dynamic covalent cross-links that exchange associatively: new bonds form before old ones break. These exchanges preserve network connectivity while permitting topological rearrangements, rendering vitrimers insoluble yet processable at elevated temperatures. Although this paradox is promising for more sustainable polymer technologies, the impact of molecular architecture on macroscopic viscoelasticity and phase behavior remains unsettled. Here, we combine theory and experiment to elucidate the fundamental physical chemistry of unentangled polystyrene (PS) vitrimer melts bearing imine cross-links. First, we study bulk PS vitrimers. We evaluate the linear viscoelasticity using small amplitude oscillatory shear, stress relaxation, and creep, then apply time-temperature superposition to the resulting data. Two regimes emerge: a fast regime governed by segmental dynamics and a slow regime exhibiting Arrhenius behavior. The slow regime shows a markedly weaker temperature sensitivity than predicted by established models for dynamic polymer networks, and is directly controlled by the cross-linker nucleophilicity and diffusivity. Second, we use a combination of experiment and hybrid molecular dynamics/Monte Carlo simulations to investigate vitrimer blends of PS and poly(ethylene glycol) (PEG). Compared with conventional PS/PEG blends, these vitrimer blends exhibit enhanced miscibility because heterotypic cross-links form between the polymers. Although imine cross-links slow the relaxation dynamics of the vitrimer blends, their rheological activation energy remains unchanged relative to that of conventional blends. This result indicates that the temperature sensitivity of the slow relaxation regime does not depend on local cross-link exchange.