Q:

Can you assume that the forces in the vertical direction are evenly distributed for an off level rigging problem?

This is an off-level rigging problem. When solving for the tension forces in the red slings (lifting from point A), can the assumption be made that the y force vector at points B and C are each half of the weight of the object? Or must one calculate the force vectors at point A? In which case the y vectors are not symmetrical. 

A:

No.
To convince yourself, why an equal distribution of the vertical component is wrong, try looking at the corresponding horizontal components (Kamran has calculated these values in his answer): The horizontal component at C is twice as large as the horizontal component at B, which results in a net horizontal load on the object, giving it a constant, horizontal acceleration off to the side. (And there is no moment equilibrium, so it would also rotate.)
I think, the easiest way to calculate the correct distribution of the vertical component is to extend the line A-C until it reaches the horizontal level of B. I call this point C'. The horizontal distance from C' to the center of gravity is $10'\cdot\frac{12'}{6'}=20'$, which is twice the distance from B to the center of gravity, so the vertical component of the reaction at B is twice the corresponding value at C or $\frac{2}{3}$ of the total weight.
Drawing the force vectors at point A and using horizontal and vertical equilibrium as you suggest in your question is also a good way to do it.

