This looks like a possible bug or perhaps an undocumented limitation. I've just tried a simple example which suggests that the Export function option AllowRasterization" -> False has the expected effect if you export 2D graphics, but appears to have no effect when used to export a Graphics3D expression.
You could use the ImageResolution option with a high DPI value (say ImageResolution->1200). This improves the resolution of the exported graphic, but it remains a raster image.
Maybe it's one for Technical Support?
Hope this helps,
Ian