A CSU will vary the pitch of the blades in flight to ensure that the prop turns at a constant speed. It is fairly common for aeroplanes to be described as having a VPP when really what is meant is that there's a CSU changing the pitch of the blades.
99% of the time CSU and VPP are one and the same.
Some kit aircraft have VPP that can be adjusted on the ground only - an example of a VPP without a CSU.
Ian