Think I understand some of your concerns, but if the glide slope gradient on an ILS is perfectly achievable for a B777, why is there a need to adopt a more shallow approach on a VSA?
When left unchecked, I've seen A320/330s and the like at 2500' with 15miles to run.
The descent gradient for either will be identical until the point of speed reduction, and then configuring, and I think you get that, hence your question.
On a VSA we may, all things being equal, decide to configure later which may have an affect on gradient.
There are usually more track miles associated with an instrument approach, track miles = altitude. Also when you program an instrument approach into the FMS it places the altitude limitations in as well, often making you higher than for a VSA. E.g. If there is a limit of 3000 ft at the IAF for an ILS, the FMS will calculate the descent to arrive at the IAF at 3000 at the appropriate speed.