a vga adapter isn't a graphics card, it's just a converter to allow your integrated video that you have on your mobo to come out of the board. likely, your integrated video can't handle vista, so as a result the passthrough (the vga adapter) doesn't work.
i'm surprised vista works at all on an fx card. they're like what, five years out of date? six?
the mobo burning out thing doesn't apply to this, it applies to two pci express x16 cards (aka, has come out relatively recently) running in sli with a bridge between them. both of those cards are PCI-only, i believe (pci 1.1, right effef?), and can't plug into a pci express slot, even if you had one.