Paul, no ignorance to your question.
First, the vinyl is stretchy. You can not see this modification, the vinyl is taut across the seat like a drum.
You can feel with your hand.
I don't think there will ever be a problem with the vinyl stretching so much that it causes a problem. If it does, I'll cross that bridge when I get to it.
I have now ridden the bike a solid week, which means about 700 miles. It could actually use a little more removal, but I will ride a bit more before I decide for sure. The vinyl does not stay stretched or sag after I get off, it simply looks like a stock seat.
If I do decide to remove any more, I'll likely buy some memory foam and try to splice it in place of the current foam while I am at it. Maybe add a heater for winter - HA!!! (surely they make a 12v heating pad for auto seats that can be modified, right?)
Dan