A vanishing point is the location in a perspective image toward which the projections of a set of parallel lines appear to converge.
The lines converge in the image
Think of the sides of a long corridor. They are parallel in the building, yet seem to meet in the distance in a photograph. That meeting point belongs to the projected image. It is not a physical point where the corridor narrows to nothing, and it is not the origin of the 3D scene.
A scene can have several vanishing points
Different families of parallel lines can lead to different vanishing points. Their positions depend on the camera orientation and projection. A set of lines parallel to the image plane does not converge at a finite point in the image. This is why the same box can be drawn in one-point, two-point or three-point perspective.
Matching a digital layer to a painting
If a virtual doorway should appear to extend a painted room, matching its apparent perspective matters as much as placing it correctly. For a Painta artwork, compare the digital layer with the painting from the intended viewing position. A painted illusion is designed for an image; an actual 3D addition changes appearance as the visitor moves. Decide whether that changing view is part of the artistic intention.