Although it is true that .vhd images aren't directly writable via dd (or for that matter cp or cat) to an internal or removable disk, the only practical difference is that they are compressed (or sparse) images. But once you have a reader (decompressor) for the format, you can recover the disk's raw contents and write them without further changes to the output disk.
Or in short, .vhd images are just compressed "dd images".
This is in contrast with ISO images, whose contents are meant specifically for CDs – e.g. using ISO-9660 instead of FAT32, and the El Torito boot format instead of DOS MBR – and Rufus has to actually transform certain parts of the contents in order to make the thing boot off a non-CD disk. (For example, it doesn't copy the ISO-966 filesystem raw, but extracts the individual files onto a new FAT32 filesystem. It doesn't always copy the bootloader as-is, but sometimes replaces it with its own version of syslinux.)
(Yes, technically ISO files are just "DD images" of a CD disc. That doesn't change the main point.)
Analogy: BMP files contain a [mostly] raw bitmap. PNG files contain heavily compressed data, but it still represents a raw bitmap when decompressed. On the other hand, DOC files don't contain anything like a bitmap and additional conversion steps are needed.