If it took the Prometheus way, the Vortex would be two times more cramped than the Prometheus itself.
Actually I tested it with the Unreal PSX Vortex Rikers too, it has changed looks a bit but in fact it is more realistic, the lava area is not the area in Nyleve though, I tried merging the maps together and it seems to fit right behind VR, but down there, as in the opposite side of nyleve, in fact the sky touched right behind the RRajigar mine entrance, implying that the lava must be from RRajigar section.
[SCREENSHOT]http://img46.imageshack.us/img46/5097/vrpsx.jpg[/SCREENSHOT]
This is right pasted in and moved, not rotated or anything (in the VR nyleve merge I had to do it by pasting it in and rotating and positioning), also it's not aligned really. SO it might not fit exactly but you get the idea.
So the question is, whether they assumed that Vortex Rikers fell in parts, or parts of it are underground etc, this seems to be case with Unreal PSX at least. But of course they changed a lot of things there, as it is common with console ports overall.
Anyway they could make it more realistic in original Unreal by making the look of the ship bigger.
EDIT: One explanation could be that once you enter vortex rikers you become dwarf, so in reality the rooms are smaller but seem bigger to you.