This extension is based off of the vala-code extension by T. Abreu (https://github.com/thiagoabreu) and the vala-grammar extension by Jereme Philippe (https://github.com/philippejer/vala-grammar).
NOTE: in order to get code intelligence, you must install the Vala Language Server (https://github.com/benwaffle/vala-language-server).
The YAML source file should be edited and then converted to XML with the "TextMate Languages" extension (https://marketplace.visualstudio.com/items?itemName=Togusa09.tmlanguage).
Note that this plugin has an issue which interacts badly with language servers (false syntax errors), so it is a good idea to disable it afterwards.