The systems manuals for the 737 state that there's a barometric input to the FPV:
The Flight Path Vector (FPV) symbol represents airplane flight path angle vertically and drift angle laterally. The flight path vector is displayed on the PFD when the EFIS control panel FPV switch is selected on. The FPV shows the Flight Path Angle (FPA) above or below the horizon line and drift angle left or right of the pitch scale's center. The FPA uses inertial and barometric altitude inputs. The vertical FPA is unreliable with unreliable primary altitude displays. (C) Boeing Aircraft Systems Manual.
This is contradicted by the QRH which states:
The flight path vector is based on inertial sources and may be used as a reference in maintaining proper path control. (C) Boeing.