Your concern is right, but you have to take a look at
http://valadoc.org/ It has full documentation on every single method and property of different calsses in all available libraries.
Also there are many good tutorials on the vala's official website which you can find it here:
https://live.gnome.org/Vala/Documentation
Let's not forget that one way to promote vala (by canonical) is to provide more sample codes and also documentation for ubuntu-specific technologies which is kind of available right now but not satisfying.
I even go further

and suggest canonical to make vala their official choice for desktop app developing. Another crazy suggestion is to create a user-friendly IDE based on vala for ubuntu. Windows has VS(maily for C#/C++ devs) and OSX has XCode(mainly for Objective-C/C++ devs) why not a new one

UDev

for Vala/C/C++ devs or maybe Python!
(I know many IDEs already exist but they all have their own issues, none of them is designed to address ubuntu specific needs)