What's required here is an insert (available from window manufacturers that make this type of vinyl window) to fill the J-channel, and then backer rod and sealant installed between the window insert and the ACMV.

A common exterior detail on homes throughout North America mixes a manufactured stone veneer, or what the industry calls adhered concrete masonry veneer (ACMV), on the lower part of the walls with some other type of cladding, such as stucco or vinyl, on the upper part. The stone creates an exterior wainscoting, if you will, that looks nice. But unless the details are executed well, it only looks nice for a short while.Builders in my area have been using manufactured stone for more than two decades, but judging from the number of failures, they still get some of the critical...