Vesicular stomatitis virus (VSV) is a member of the order Mononegavirales, which consists of viruses with genomes of nonsegmented negative-sense (NNS) RNA. Many insights into the molecular biology of NNS viruses were first made in VSV, which is often studied as a prototype for members of this order. Like those of other NNS viruses, the VSV RNA polymerase consists of a complex of the large protein (L) and the phosphoprotein (P). Recent discoveries have produced a model in which the N-terminal disordered segment of P (PNTD) coordinates the C-terminal accessory domains to produce a “compacted” L conformation. Despite this advance, the role of the three phosphorylation sites in PNTD has remained unknown. Using nuclear magnetic resonance spectroscopy to analyze the interactions between PNTD and the L protein C-terminal domain (LCTD), we demonstrated our ability to test sensitively for changes in the interface between the two proteins. This method showed that the binding site for PNTD on LCTD is longer than was previously appreciated. We demonstrated that phosphorylation of PNTD modulates its interaction with LCTD, and we used a minigenome reporter system to validate the functional significance of the PNTD-LCTD interaction. Using an electron microscopy approach, we showed that L bound to phosphorylated PNTD displays increased conformational heterogeneity in solution. Taken as a whole, our studies suggest a model in which phosphorylation of PNTD modulates its cofactor and conformational regulatory activities with L. IMPORTANCE Polymerase-cofactor interactions such as those addressed in this study are absolute requirements for mononegavirus RNA synthesis. Although cofactor phosphorylation is present in most of these interactions, its effect, if any, on this protein-protein interaction had not been addressed. Our study is the first to address the effects of phosphorylation on P during its interactions with L in residue-by-residue detail. Since phosphorylation is the biologically relevant state of the cofactor, our demonstration of its effects on L conformation suggests that the structural picture of L during infection might be more complex than previously appreciated.