I've done several Vanagon windshields and others too. To make that
seal pliable, I just lay it out in the sun. To make things even
easier, I treat the lip on the body with silicone lubricant and some
on the seal mating surface. Here is where some debate might arise. I'm
not saying this is right, but where I learned the little I know of
windshield replacement, 'roped' windshields, like on the Vanagon, get
zero black poly. No sealant whatsoever, using new seals, of course.
This also eliminates removal of hardened sealant during the next
replacement....and you know many can't resist the urge to use a razor
blade, which more often than not, results in scarring the paint and
exposing the metal.
Footnote : I used to get called into this windshield shop, from time
to time when they were busy. It was 100.00 in my pocket for every
windshield so it was easy cash during harder times. The owner of the
shop was one of these guys who could not be trusted with that black
poly. Every day I worked there, he had to scramble to remove that
black crap from someone's nice upholstery, or he would lose trim, or
break something. I watched it become a place of ill repute and never
returned.
Miguel

He who possesses art and science has religion; he who does not possess
them, needs religion.
Johann Wolfgang von Goethe