Update on the Van-tek windows -
The gap is normal and allegedly present on all of the available half sliders out there. I was told that I was pulling the sliding glass back too far which exposes the gap. My concern was that not pulling the glass back far enough results in the latch being unable to hold the screen in place. This allows anyone outside the van to slide the screen open and reach inside. I was told this was normal and that the screen was not supposed to be latched when the window was open.
I cannot speak for other windows on the market, but if it is true that all companies have similar issues, Van-Tek windows are fine. If the gap/security issue is unique to Van-Tek, I would buy from a different company.
I will post a full review on my website with the updated build hopefully this week. More photos and such.