The structural, elastic and lattice vibration properties of the ternary alloys ${\rm YP}_{1-x}{\rm As}_{x}$ at various As concentrations ($x$) from 0 to 1, $x=0, 0.25 ,0.5, 0.75$ and 1, are presented. The calculations were performed using the density functional perturbation theory (DFPT) within the generalised gradient approximation (GGA) and employing virtual crystal approximation (VCA). We studied the effect of arsenic composition on structural parameters, the phase transition pressure, the elastic constants, the optical and acoustic phonon frequencies at high symmetry points $\Gamma$, $X$ and $L$, the static and electronic dielectric constants, and the Born effective charge. It is established that all these properties follow a quadratic law in the concentration ($x$). There is a good agreement between our results and the available data for binary compounds YP and YAs. Our results are predictions of ${\rm YP}_{1-x}{\rm As}_{x}$.