There is a new pull request by sgn against master on the void-packages repository

https://github.com/sgn/void-packages valac-vapidir
https://github.com/void-linux/void-packages/pull/28111

valac's wrapper vapidir fix
Alternative, we can use this hack http://ix.io/2MZp

A patch file from https://github.com/void-linux/void-packages/pull/28111.patch is attached