  We study Sr$_2$(Ba$_2$)VO$_4$ under high pressure by means of the local
density approximation + dynamical mean field theory method. While Sr$_2$VO$_4$
is a 1/6-filling three-band system at ambient pressure with a small level
splitting between the $d_{xy}$- and $d_{yz/zx}$-bands, we show that an orbital
polarization occurs under uniaxial pressure, resulting in dramatic changes of
the magnetic, optical, and transport properties. When pressure is applied in
the $c$-direction, a $d^1$ analog of $d^9$ cuprates is realized, making
Sr$_2$(Ba$_2$)VO$_4$ a possible candidate for a $d^1$ superconductor.
Experimentally, this uniaxial pressure can be realized by growing Ba$_2$VO$_4$
on a substrate with lattice constant 4.1-4.2 \AA.
