The increasing demand and rising cost of fossil fuels, coupled with the depletion of their reserves and the growing environmental pollution they cause, have motivated researchers to explore alternative fuels. Among these alternatives, biodiesel has emerged as a promising renewable option. Given the ongoing fossil fuel crisis and the numerous advantages of biodiesel, its widespread adoption in the near future appears inevitable. Therefore, it is crucial to investigate the effects of biodiesel–diesel blends on engine performance and environmental emissions. This study experimentally examines the impact of various biodiesel–diesel blend ratios on the performance parameters, exhaust emissions, and vibration characteristics of a diesel engine. Biodiesel was produced from waste cooking oil through the transesterification process and blended with conventional diesel at volumetric ratios of 10%, 20%, 30%, 40%, and 50%. Pure diesel was used as the reference fuel. The performance, emission, and vibration characteristics of these blends were evaluated in accordance with the ECE-R49 standard (constant speed mode) at engine speeds of 1500 and 2200 rpm under four load levels: 25%, 50%, 75%, and 100% of full load. The results indicate that increasing the biodiesel fraction in the blend reduces the emissions of Carbon Dioxide (CO₂), Nitrogen Oxides (NOₓ), and soot, while increasing the oxygen content in the exhaust gases. However, no clear trend was observed for unburned hydrocarbon emissions. Engine torque and power remained nearly constant with increasing biodiesel content, whereas exhaust gas temperature decreased and the lambda value increased. Time-domain vibration analysis revealed that vibration levels increase with engine speed and load, whereas higher biodiesel content in the fuel blend leads to reduced vibration and noise levels. Frequency-domain analysis further showed that the dominant vibration frequencies for all fuel samples lie in the range of 0–320 Hz. The peak vibration frequencies varied with engine speed but remained unaffected by biodiesel ratio or engine load; however, vibration amplitudes decreased with higher biodiesel content and lower engine load.