Hi! Sorry for the strange issue but I'm having trouble figuring out how I would add compatability, I don't think it's doable with a datapack? I'm honestly not entirely certain how to even add blocks to the windowable tag since I've avoided editing datapacks since they were added, but in either case this mod presents a bit of an issue since vertical slabs from Just Vertical Slabs are generated at runtime and there's an arbitrary number at any given time as a result, so each block couldn't be added to the tag individually.

I see Quark and Double Slabs Vertical Slab compat was added manually to the tag by putting their respective vertical slab tags into the windowloggability tag but JVS doesn't actually have a tag with all its vertical slabs inside unless I missed something so I'm not certain how compat would be added... Nor if things would even render correctly if it was due to how JVS generates its textures and models.

If there's an easy way to add compat myself I'll do that, but if not I'd love to see compat added, since currently it feels like a disadvantage to use this mod in particular compared to other vertical slab options, despite others not having vertical slabs for every horizontal slab variant without extra compat mods which might miss things.

its two block IDs
justverticalslabs:cutout_vertical_slab and justverticalslabs:translucent_vertical_slab

Just adding those two should in theory work

Does the datapack show up with the /datapack list command and is it enabled?
(if its not enabled but listed, you can run /datapack enable ...)

Hmm... I'm probably doing something obvious wrong but that didn't seem to work.

What I did was I made a datapack (zip folder) and inside it there's the folder structure data\windowlogging\tags\blocks\ then windowable.json inside that, which contains this, based off the implementation of the data in Windowlogging's jar:

{
  "replace": false,
  "values": [
    { "id": "justverticalslabs:cutout_vertical_slab", "required": false },
    { "id": "justverticalslabs:translucent_vertical_slab", "required": false }
  ]
}

But upon putting it in a save's datapack folder and launching the game, I try to place a window inside a vertical slab and nothing happens still.

Am I doing something obvious wrong?

Whoops, since I was following the folder structure of the data in the jar folder I forgot external packs need a pack.mcmeta file.
That said, it works now... Sort of. As I was kind of dreading, I guess because of the way JVS defines what block type the slab is, this happens whenever I windowlog one:

I suppose at this point I should report this to the author of JVS?

Well at this point its on me to respect models that already went through model baking. There is probably not too much the JVS Dev can do here. You can try reporting it, but it is pretty likely I am the one who needs to fix it. Which may take some time as i have exams coming up.

Ahh, that's okay though, it's no rush.
Honestly I wasn't expecting it to work in the first place since both these mods do what seems like impossible witchcraft.

I'll just have to reconsider some of the builds I was thinking of doing or scale them up or something.
Thanks for the help though! It's appreciated! And good luck with your exams!