I've recently installed VSCode version 1.29.1-1542309157 using snap on Ubuntu and I've installed Fira Code and Hack.
But it doesn't seem to work. I've restarted the OS for a few times and it still doesn't work. It will only read the fonts that have been previously installed in the system like 'Ubuntu Mono'.
The issue seems to be the restrictions that snap imposes to apps installed with it. I had the exact same problem, then I uninstalled VSCode and reinstalled it with a .deb
and everything just worked like a charm. There is an issue in the VSCode repo, in case you want to follow it: https://github.com/Microsoft/vscode/issues/63194.
By the way, thank you so much for mentioning you had installed it with snap. That's what made it click for me.