One of the best adhesives for what I believe you guys are trying to repair is a 3M product: 3M™ Channel Bonding and Sidelite Adhesive
It really is an amazing product, fairly easy to use IF one follows the instructions.
It does resemble "normal" epoxy however its specially formulated for automotive use. Exposure to elements, constant vibrations and materials to be bonded are factors 3M used designing this product
Its downside is its 3M meaning its costly and in this case pretty much requires using their suggested tools for proper application and use. Even the least expensive of those are a bit expensive for a one time application but since this would eliminate replacing the entire window acquiring the whole system might be cost effective.
Any further questions please ask----I'm sure this isn't a pleasant prospect if it happens to an already-finished or built-out van.