The kinematic vorticity number is an important quantity in structural geology and tectonics, giving a nonlinear ratio of simple shear to pure shear deformation. We use natural observations and numerical models to show how rigid clast methods for determining the kinematic vorticity number (Wk) are compromised where strain localization occurs at the matrix-clast interface. Our numerical results show that the critical shape factor cutoff between permanently rotating and stable clasts, used to determine Wk, is highly sensitive to coupling between the clast and the matrix. This finding provides an elegant explanation for the fact that rigid clast methods tend to underestimate Wk relative to other methods. We present numerically determined envelopes for clast behavior across a range of kinematic vorticity numbers, clast shape factors, and matrix-clast coupling. Our numerical models show that the shape-preferred orientations of feldspar clasts trend toward the positions of mica fish with increasing localization at the clast boundary, suggesting that mica fish behave as highly lubricated clasts. Our data and numerical results show that the clast-matrix interface may be several orders of magnitude weaker than the surrounding matrix and that weak interfaces can lead to a marked drop in the bulk shear strength of faults and shear zones.