You should probably make it clear that this command will only write to pre-existing filesystems on the USB devices that U-boot has compiled in support to write.
save <interface> <dev[:part]> <addr> <filename> bytes [pos]
- Save binary file 'filename' to partition 'part' on device
type 'interface' instance 'dev' from addr 'addr' in memory.
'bytes' gives the size to save in bytes and is mandatory.
'pos' gives the file byte position to start writing to.
If 'pos' is 0 or omitted, the file is written from the start.
In the case of the VF2(/?VF2L?) it looks like Das U-Boot currently supports FAT (which would include FAT32), ext2 and ext4 filesystems.