Tool mkfs.(v)fat is provided by package dosfstools and in OpenWrt commit [1], it was merged to single package. Because of that mkfs.vfat is no longer provided.
Let's use mkfs.fat.

[1] https://github.com/openwrt/packages/commit/5d032bce1f0eb259ba94e7d8bed1444a1236f8b0#diff-4ae5696ed12100b6b90f3e12991fcd054b35f4fbb3af426d5bacdda6f2261948