The perspective grid is a crutch. When your drawing gets more complex than a mere box you can not rely on a grid anymore. A detailed scene might need a hundred or so grids. But a lot of art can still get a lot of mileage out of restricting themseves.
Instead of a grid start constructing your scene by projecting items. This allows you to build a wanishing ...
When you rotate a 1 point perspective box that is parallel to the projection plane (eye or camera) in one axis, it becomes 2 point. However, the smaller the angle of rotation, the less obvious the convergence between parallel lines. At 45 degrees the planes in a cube will mirror each other.
Notice that the cube is intersecting the horizon line at the ...
I realize this is an old question, but worth a revisit I think.
You can find the vanishing points in an image by extrapolating lines of perspective within it.
You can draw simple lines in Inkscape and turn them into guides by hitting Shift+G.
This example image has two vanishing points:
Using the room itself, it would not be correct. Since there are two angles to use within the room, using those would provide more accurate perspective based upon the existing angles in the room (that I can see).
I have no idea how the horizon in your image was determined. There may be other aspects outside your posted image which warranted horizon placement....