Bah! I wasn't using the newest vibe.d version. Now the compile doesn't fail anymore (it did before, saying the template didn't start with "!!! 5"; forgot to mention that).
But, instead, the blocks filled in the rendered page aren't filled in the result.
Sorry for the last example, I've since noticed the syntax was wrong in a few parts and have updated them. I'll also write a smaller example here, to hopefully outline my intentions better.
!!! 5 html head block title link(rel="stylesheet", type="text/css", href="css/bootstrap.min.css") body block body
extends layout block body .modal .modal-header block modal-header .modal-body block modal-body .modal-footer block modal-footer
extends layout_modal block title title Test page block modal-header h2 Test Page block modal-body p Some kind of form will be put here.
So, the result of this is as if I were rendering layout_modal.dt directly. Seems like login.dt is ignored? (The title doesn't get set either.) I'm 100 % certain that I'm actually calling .render!("login.dt") and not layout_modal.dt.