Using a high-resolution (Delta E approximate to 1 meV) laser photoelectron attachment method, we have studied cluster anion formation in collisions of low-energy electrons (1-180 meV) with (N2O)(N) clusters. We show that formation of both heterogeneous cluster anions (N2O)(q)O- (q < N) and (as a minor channel) of homogeneous anions (N2O)(p)(-) (p less than or equal to N) is mediated through narrow, weakly red-shifted vibrational Feshbach resonances of the type [(N2O)(N-1). N2O(nu(1))](-) (nu(i) denotes vibrational excitation). For the first time direct information has been obtained on the relationship between the size q of the heterogenous anions and the size p of the homogeneous anions (p-q approximate to 2-3 for p = 8, 9, 11, 13), as formed from the same precursor cluster size involved in the vibrational Feshbach resonance. The electron-cluster binding energies are recovered from model calculations based on appropriate short- and long-range forces.