I feel for you, Gary. F1 says that the VNAV function "isn't supported", which is another way of saying they just didn't model it into the plane.
While I don't have any problem with the autothrottles being absent (the real plane doesn't have them either), I sure wish they would have made the plane complete by including the VNAV function (as the real plane does).
The problem with utilizing ISG gauges for their VNAV function is that they generally won't "talk" to your existing guages and you'd be stuck planting the MFD into the Mustang panel, which would be rather odd, IMO. Nor can you get an ISG FMS to paint a BOD/TOD on your G1000.