The significance of the precipitation evolution and interaction between the nanoscale MC carbides and dislocation in a novel high-strength weld metal with different V contents following post-weld heat treatment (PWHT) at 640 °C for 2 h durations were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), and atom probe tomography (APT). Quantitative calculations of the strengthening contributions were performed by considering the solute atoms, subgrain boundaries, dislocations, and precipitates. According to the TEM observations and extracted XRD results, the volume fractions of the M2C and M3C carbides gradually decreased, while the volume fraction of the nanoscale MC carbides increased with an increasing V content. The APT results indicated that the nanoscale MC carbides primarily contained V and Mo. The precipitation strengthening could be attributed to the fine M2C carbides and discrete MC carbides being randomly distributed in the bainite in the weld metals without V and with 0.1 wt% or 0.18 wt% V, respectively. Numerous nanoscale MC carbides may act as obstacles to inhibit dislocation annihilation during the PWHT, resulting in lower impact toughness with V addition.