A vanadium self-powered neutron detector has been used in recently designed Westinghouse four-loop pressurized water reactor and AP1000^® plants as the in-core instrumentation system detector. Vanadium has a smaller cross section when compared to rhodium and platinum self-powered neutron detectors, which allows it to have a longer lifetime in the reactor core. A vanadium detector is also known to have a measurable component of gamma response in its detector signal in addition to desired neutron response, which may prevent it from performing a load follow and online core monitoring/surveillance function for the advanced modern reactor design if this gamma response component becomes too large. In order to investigate this phenomenon, a series of detailed pressurized water reactor fuel assembly Monte Carlo N-Particle models has been built to calculate the gamma contribution fractions in fuel assemblies with various initial enrichment and burnup levels. Several experiments have been performed at the Penn State University Breazeale research reactor to measure the gamma and neutron sensitivity of the vanadium in-core detector. The gamma contribution fraction has also been measured at the Penn State University Breazeale reactor by scramming the reactor from full power and by various power maneuvers. Similar tests have also been performed at a Westinghouse four-loop reactor with a vanadium fixed in-core instrumentation system. All the results indicate that the gamma contribution fractions in the vanadium in-core detector designed by Westinghouse for commercial pressurized water reactors will perform its designed functions.