In recent decades, there is growing interest in various applications of polymerized ionic liquids (PILs), particularly in the aqueous biphasic systems (ABSs) used as media for extraction of solutes. In this work, we report new experimental data on the binodal curve and tie lines in the ABSs containing PIL and non-polymerized ionic liquid (IL): poly-[C4Vim]Br-K3PO4-H2O and [C4Vim]Br-K3PO4-H2O at 298.15 K. For the ABS with PIL, the partition coefficient of L-tryptophan between the liquid phases is obtained from experiment and compared with our previous data on ABS [C4mim]Br-K3PO4-H2O containing non-polymerized IL. We conclude that the ABS containing poly-[C4Vim]Br has a lower extraction efficiency than the ABS based on non-polymerized ILs. To elucidate the mechanism underlying such behavior of the PIL-containing mixture, we performed MD simulations of PIL-rich aqueous mixtures in presence of K3PO4 and L-tryptophan. We obtained diffusion coefficients of low molecular mass ions and water, and data on the distribution of these species around polycation over a range of mixture compositions. MD data show that poly-[C4Vim]Br exhibits no favorable selectivity towards L-tryptophan anions in the presence of phosphate background. This confirms unfavorable combination of poly-[C4Vim]Br with phosphate for the extraction of L-tryptophan, in accord with our experimental findings.