Herein, we employed in situ small-angle neutron scattering (SANS) and neutron reflectivity (NR) to elucidate the importance of polymer-solvent interactions on morphology development during solvent vapor annealing (SVA) of block polymer (BP) thin films. Judicious choice of nanostructure orientation (parallel to the substrate) permitted measurement of the preferential segregation of solvent (d(6)-benzene) into our cylinder-forming poly(styrene-b-isoprene-b-styrene) (SIS) films. A distinguishing feature of this work is the simultaneous tracking of nanostructure evolution and solvent segregation, enabled through the combination of SANS (in-plane features), NR (out-of-plane features), and solvent deuteration, which directly related polymer-solvent interactions to morphology reorganization. We found that at higher d(6)-benzene partial pressures (p/p(sat)) the number of cylinder layers (domains) increased or decreased to accommodate overall film thickness changes upon swelling/deswelling, while the SIS domain spacing remained nearly constant. However, at lower p/p(sat) values, the SIS domain spacing changed to accommodate similar swelling/deswelling variations, while the number of layers remained invariant. The threshold for this p/p(sat) transition was directly related to plasticization of the polystyrene (PS) block, which has significant implications on the size of the domains, degree of ordering, and interfacial roughness. Thus, by linking polymer-solvent interactions to morphology evolution, we have developed an improved understanding of the interplay between the kinetic and thermodynamic effects that can direct the self-assembly and through-film periodicity of nanostructured thin films