The fact that the D800 files are less saturated than your slide film is normal, especially if you use RAW, and especially if you happen to use saturated slide film like Velvia 50. The D800 has a lot of dynamic range (more than slide film which we know is difficult to expose in contrast scenes). Nikon had to fit all that extra highlight and shadow data into the RAW file, data which the slide film does not have, with the result that the RAW image by default may not look as punchy.
So this is by intention. The RAW file with all this data now gives you a huge margin to work with, to change the curves, contrast, saturation etc in post processing. You can easily bring it up to the slide look and way beyond. Programs like Lightroom make this quite easy. But if you don't have much experience with this yet, another option is to use JPEG, and choose a picture control like Vivid. Since the D800 has two card slots, a good way to do this is to set the camera to NEF + JPEG, and configure slot 1 for NEF and slot 2 for JPEG. That way you have both - the JPEGs for immediate, good looking results, and the RAW for more intricate work later as you gain practice.
Regarding the vertical (portrait) orientation, there is no way the images are cropped any differently than in landscape orientation. So what you're seeing is probably an illusion. The D800 LCD does NOT have the same aspect ratio as the full size image, so seeing black borders is normal.