I save as a photoshop file and then save as a color BMP file. Apparently a bitmap will keep crisp edges and not soften them like Jpeg's, etc
Lionel, just for clarification both BMP & jpeg are bitmap formats. The difference between the 2 is how they compress a file, a jpeg gives a higher compression resulting in a much smaller file size, it does this by deleting a lot of the colour information which isn't really needed if you are viewing on a monitor.
BMP & Tiff retain a lot more of the original colour information but produce a much larger file, if you are enlarging prints then they are much better file formats to start out with, this is most likely the reason for the better results you see in PVC.
Tiff is another format that works very well for large format printing, as you still seem to be experimenting a bit then it might be worth trying a tiff file to see if you get any better results although looking at your pictures you may not see any difference at all