I think it has to be type specific.
Vy corresponds to the maximum "excess power", i.e. the airspeed at which the power produced by the engine is furthest above the power required to overcome drag. Both powers vary with airspeed and so Vy depends on the exact shape of the two curves.
Generally speaking, maximum engine power decreases with density altitude as the cylinders get filled with less dense air, i.e. less fuel can be burned with the air available. There are propeller effects too. Obviously this changes with turbo charging and wobbly props.
Power required is an aerodynamic force, (whole aircraft Drag), times the true airspeed. For any given Indicated airspeed and weight, Drag should not change, but the True airspeed rises with density altitude, and so the Power required does too.
This is covered (with diagrams) in PPL ground school books.
Last edited by 24Carrot; 20th June 2013 at 12:01.