The Venus 2000 is a stroking machine, not a sucking machine. It's cleverly designed such that all air in the liner gets pushed out a one-way valve by the penis, until the liner is held up tight against the skin, but no vacuum is ever actually applied inside the liner itself. Alternating positive and negative pressure is applied outside the liner to cause it ride up and down an erect or flaccid penis.
A milking machine works by applying only vacuum and alternating it between the inside and the outside of the liner.
There are lots of posts around the web about milking machines being used for masturbating, but they will cause injury if the vacuum isn't seriously reduced. Also, it is common to modify the pulsator to a more desirable stroking/sucking pattern.