It's a symlink to busybox, so it's part of busybox, you can also try to run it using 'busybox mkfs.vfat' optionally:
shell@android:/ $ busybox mkfs.vfat
BusyBox v1.20.2-jb bionic (2012-07-27 02:57 +0200) multi-call binary.
Usage: mkfs.vfat [-v] [-n LABEL] BLOCKDEV [KBYTES]
Make a FAT32 filesystem
-v Verbose
-n LBL Volume label