A detailed investigation of vortex nucleation in He II containing traces of 3He isotopic impurity is reported. Systematic measurements of the nucleation rate due to negative ions were made at 23 bars. A model is proposed which, when fitted to the data, implies that a single 3He atom trapped on the surface of the ion has the dual effect of reducing the critical velocity for vortex nucleation by circa 4 m/s, while simultaneously increasing the corresponding rate constant by a factor of circa 10^3.