My guess is that you left the LVDS in your port list and have it as the first port. The driver has to assume that an LVDS is attached and will make it the primary video.
A couple of things could help here-
1. change the port order to have the SDVO first. Also you could consider using "display detect" so the driver will look on the SDVO port for a valif display attached. This is best and least problematic solution.
2. Set CLONE mode as default startup mode and that way BOTH the LVDS and SDVO will be primary and you will get video on the VGA. The downside to this is that you will potentially limit your display modes.
If the above does not change anything then it may also be that the driver is NOT seeing a valid configuration (installation falied to update the registry due to a build issue or ???) and is loading but going into its default mode. It could also be that your VGA display says it handles 640 x 480 mode or 800 x 600 but does not (Windows sometimes starts up VGA displays in the lowest resolution possible- some displays do not handle the modes they "say" they do through their EDID). You could see by attaching an LVDS and a DIFFERENT VGA display to see what might be going on.