Re: from 8 to 16 bit vga?
There is only one pin for each color (red, green blue) in a VGA connector.
These signals are not digital. Somewhere between the FPGA and the connector the digital signals must be converted to analog by a D/A converter.
In the Nexys 3 board, this is done by a few resistors for each color. Doing a 2-3 bit D/A-converter this way is easy.
An 8-bit D/A-converter requires much better resistors and of course more pins on the FPGA.
It may be easier to buy a chip made for this purpose.