The image pixel data contained in raw TIFF and DNG files produced by VueScan is identical. This can be proved by extracting and comparing that data. This can be done using Linux dcraw and imagemagick command-line tools:

$ diff -s \
 <(stream -map rgb -storage-type short raw.tif - | sha256sum) \
 <(dcraw -E -4 -c raw.dng | tail -n +4 | dd conv=swab 2> /dev/null | sha256sum)
Files /dev/fd/63 and /dev/fd/62 are identical

(I won't muddy this answer with a technical explanation - add a comment if that would be of interest here.)

The overall file content, however, is different. A DNG file is a TIFF file with a specific structure that includes a low-resolution preview and special metadata. Of particular note are the following tags:

• ColorMatrix1
• As Shot White XY
• Baseline Noise

These three tags are set by VueScan and used by a developer application (such as DarkTable) as an input colour profile (which DarkTable lists as embedded matrix). There are some other tags but they don't affect how the image is processed.

You can take a VueScan DNG and "re-scan" it into a raw TIFF and the pixel data in both files will be the same. You could alternatively use dcraw to do the same thing: $ dcraw -E -4 -T raw.dng

So the issues raised in the question are not so much due to the differences between the file contents but how Darktable (or whatever) processes the different files. You can make a DNG look like a TIFF by removing the abovementioned tags and selecting the sRGB colour space.

With regard to file sizes - I haven't looked at the effectiveness of compression algorithms because it would appear to be, as others have suggested, dependent on the distribution of the input data. But, comparing like-for-like, the image bitmap data content is identical.

What remains unclear is how VueScan computes ColorMatrix1, AsShotWhiteXY and BaselineNoise.