Nonlinear viscoelastic properties are reported for composites of fumed silica with various surface treatments and matrices of poly(vinyl acetate) of different molecular weights as well as a copolymer matrix of vinyl acetate and vinyl alcohol. Data above the glass transition temperature are reported here. The increase in the composite storage and loss moduli measured at low strains, and their relative rates of decrease with strain, are found to depend on filler surface treatment. The nonlinear behavior of the loss factor with strain is dramatically altered by filler treatment and quite revealing as to the likely mechanism causing the nonlinearity. In addition, the relative reinforcement and the degree of nonlinearity are found to be the highest for the lowest molecular weight matrices. The effect of copolymer substitution for the homopolymer matrix is equivalent to an increase in molecular weight. The primary underlying mechanism for reinforcement and nonlinear behavior appears to be the filler-matrix interactions, but not filler agglomeration or percolation. It is proposed that temporary (labile) bonding of chains to the filler surface results in trapped entanglements, having both near- and far-field effects on matrix chain motions. These trapped entanglements cause greatly enhanced non-Gaussian (Langevin) chain behavior that affects storage and loss moduli differently, resulting in very high reinforcement by nanofillers. Applied strain (stress) aids the release of the trapped entanglements, thereby leading to the reduction in dynamic moduli. The reinforcement and nonlinear viscoelastic properties of the nanofilled polymer melts bear striking similarity to what is observed in filled elastomers (the Payne effect), suggesting a common mechanism that is rooted in the macromolecular nature of the matrices.