I've never used a VLab Motion before, but something tells me that you shouldn't be hooking it up to the SCSI chain, but rather giving it its own chain. That's how other video cards work, at least.
It doesn't seem like it would make sense to attach it to the existing chain since it should already be able to access the existing chain through the Zorro interface.
One last point: are you absolutely certain that it's a SCSI port on the card? There are a few cards from other manufacturers (DPS) that have 50pin headers that are not SCSI ports.