Hi Pegart, i have no knowledge of custom boot animations but here is what i know.
Samsung Galaxy i9000 is very different from other android phones in the way the system is. The boot animation pngs you see are the shutdown ones and you could change it. With SGS tools (available free in market) you can change boot sounds. The startup is in qmg in which there is no way to change yet. over in the vibrant forums they are working on it and i think they have done it and you should ask in the vibrant forums how they done it which might help you
Hope this information helped you