Controlling the morphology, domain orientation, and domain size of block copolymer (BCP) thin films is desirable for many applications in nanotechnology. These properties can be tuned during solvent annealing by varying the solvent choice and degree of swelling which affect the effective miscibility and volume fraction of the BCP domains. In this work, we demonstrate with a bulk lamellae-forming BCP, poly(4-trimethylsilylstyrene-block-D,L-lactide) (PTMSS-b-PLA), that varying the composition of a mixture of solvent vapors containing cyclohexane (PTMSS-selective) and acetone (PLA-selective), enables formation of perpendicularly oriented lamellae with sub-20-nm pitch lines. The BCP domain periodicity was also observed to increase by 30%, compared to bulk, following solvent annealing. Furthermore, solvent annealing alone is shown to induce a transition from a disordered to an ordered BCP. We rationalize our observations by hypothesizing that the use of a combination of domain selective solvent mixtures serves to increase the effective repulsion between the blocks of the copolymer. We furnish results from self-consistent field theory calculations to support the proposed mechanism.