This game has hidden developer credits.
Vigilant is one of many mediocre space shooters.
As with many Nice Code games, pressing Up, Down, Up, Left, Right, Left, B on the title screen will briefly display a credits screen. Nature Color Game is one of several aliases used by Nice Code, and PJ most likely refers to the Power Joy series of Famiclones which many Nice Code games have been released on.
An alternate VT02 version of this game exists which changes the graphics, adds music, removes the 2 player option and disables the code to access the credits screen.