I think you mean Template Variables rather then Variable Templates (no such thing).
If the default value of the TV is @INHERIT, when you enter a value in the TV, that Value will be inherited by all that resource's children (assuming that they don't already have a value set for the TV).
If you want the value to be available in *all* other resources, not just the Children, you don't want a TV. TV values are intended to be for specific resources.
Instead, you probably want a create a new System Setting. When the System Setting has a value, that value will be available all over the site with a tag like this:
If you need an easy way to set the System Setting, try Mark Hamstra's excellent ClientConfig extra.