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 water-management details wrong. While there are still problems with flashings and drainage planes on the upper cladding, the basic materials to create a drainage plane are in place. But builders are treating the exterior as if the siding stops at the stone, as if the lower sections of wall are covered by an impervious material that will completely protect the wood framing to which it is adhered. To the contrary, manufactured stone is very porous and will absorb water. Think of it as fat stucco with chunks of concrete in it. That's really what it is, and it should be detailed accordingly.
I was recently called in to look at a house sided with vinyl siding and manufactured stone. Water staining on the interior drywall and leakage below a window in the garage prompted the call, but inspection also revealed efflorescence on the stone, damage to the OSB near the foundation sills in other parts of the house, and corrosion, even rot, around fasteners through the sheathing.
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