I'm rendering large amounts of sprites that have the same y orientation.
In other words, the lower z values are all 0, and the higher y values are all 100 (or some other constant value).

For example, imagine a city map aligned to the x/z plane. A large number of sprites are overlaid on top of this map. So my geometry is in essence planar (I only have lat/lon, in other words).

Is it possible to have memory, and only encode two of the coordinates (x and z) of each point in the VBO, and supply a constant y coordinate value in addition to that?

This would reduce my memory requirements by 33%, obviously.