Now, I happened across a thing such as mixin(import("user.d")) which is less elegant, but would work great for me. But I'm getting this error for relative and absolute paths to files:

source/bolt/bolt.d(109): Error: file "user.d" cannot be found or not in a path specified with -J

Solution was easy here. Had to add stringImportPaths to dub.json:

{
"name": "bolt",
"description": "A simple vibe.d server application.",
"copyright": "Copyright © 2015",
"authors": [""],
"dependencies": {
	"vibe-d": "~>0.7.19"
},
"versions": ["VibeDefaultMain"],
"stringImportPaths": ["views/", "source/bolt/views"]
}

Have to keep "views/" in the list, or else links to the diet templates will break.