Our subject of interest is the coupling between the rheological, macroscopic properties of a complex fluid and the dynamics of its constituents on a micro-scale, such as vesicles in a suspension. Hydrodynamic interactions between soft micro-objects in a concentrated suspension introduce additional complexities to this micro-macro link. Our experiments on vesicle pair interactions in a linear flow, realized in a microfluidic four-roll mill and supplemented by measurements of a single vesicle back-reaction on the velocity field, show that a characteristic spatial scale for vesicle-vesicle interaction is ~3.5 times the effective radius of a vesicle. From this result we estimate an upper bound for the volume fraction, above which a suspension cannot be considered dilute.