For the last week or so, every .VUE file I work with has no syntax highlighting. I usually have the extension Vetur to help with synthax highlighting and other tweaks to .VUE files.
Things I've tried so far:
As i see for lang="pug" there is no needs to use other extension as like below:
https://vuejs.github.io/vetur/highlighting.html#custom-block
So I think the problem may be on your project file, please check new vue project to be sure that the problem is not from your files and config of the project !