By associating a Grid with a File Extension, that Grid will be automatically loaded whenever a file with that extension is viewed.
To do this, simply create a Grid, and when saving it, make sure that the name of the Grid begins with the file extension.
For example, to associate a Grid with .xyz files, simply name the Grid something like ".xyz (Grid for XYZ Files)". Any description may follow the file extension (as long as there is a separating space).
You must also enable Auto-Load Grid Extensions on the GridLines menu.
A File Extension Grid must be saved on the GridLines menu. You cannot save it as a .vgrid file in the current directory or the Default Grid Directory.