In solid-state chemistry, the direct reaction of elements at low temperatures is limited by low solid-state interdiffusion rates. This and the limited number of processing parameters often prevent the synthesis of metastable compounds. Precisely controlling the number of atoms and nanoarchitecture of layered elemental precursors enabled the selective synthesis of two closely related metastable tin vanadium selenides via near-diffusionless reactions at low temperatures. Although the nanoarchitectures of the precursors required to form [(SnSe2)(0.80)](1)(VSe2)(1) and [(SnSe)(1.15)](1) (VSe2)(1) are very similar, controlling the local composition of the SnISe layers in the precursors enables the selective synthesis of either compound. The metastable alloy SnxV1-xSe2 was preferentially formed over [(SnSe2)(0.80)](1)(VSe2)(1), which has the identical composition, by modifying the nanoarchitecture of the precursor. Ex situ in-plane X-ray diffraction and X-ray reflectivity collected as a function of annealing temperature provided information on lateral and perpendicular growth of [(SnSe2)(0.80)](1)(VSe2)(1). The presence of Laue oscillations throughout the self-assembly provided atomic-scale information on the thickness of the [(SnSe2)(0.80)](1)(VSe2)(1) domains, giving insights into the self-assembly process. A reaction mechanism is proposed and used to rationalize how composition and nanoarchitecture control the reaction pathway through the free energy landscape.