for example, if a constructor has a parameter like :
using Clutter;

....

public AClass(AnimationMode anim_mode = AnimationMode.LINEAR) {
...
}

then, the generated vapi file for this library will be :

public AClass(Clutter.AnimationMode anim_mode = AnimationMode.LINEAR);

which fails because AnimationMode can't be found in the context of "AClass"

the workaround consist in writing the default value with the full namespace in vala :

public AClass(AnimationMode anim_mode = Clutter.AnimationMode.LINEAR) {
...
}

which generates the right vapi.