As per the MFG Tool Documentation, Initially board will boot with uImage_mfg and enumerate as UMS device, then copies all images for NAND to DDR. After copying to DDR it will write all images to NAND Flash.
How MFG Tool will know the NAND partition info? so that it will write all images(U-boot, Dtb, zImage and rootfs) to NAND flash. The .vbs file is mentioning only the partition numbers.