installed Ventoy
the folder structure is
grub/grub.cfg
EFI/BOOT/
I would use chainloader to add to grub try:
menuentry 'Ventoy Boot USB' {
insmod part_gpt
insmod ext2
insmod fat
insmod exfat
insmod ntfs
insmod chain
search --no-floppy --set=root --fs-uuid 440A-1007
chainloader /EFI/BOOT/BOOTX64.efi
}
or chainloader /EFI/BOOT/grub.efi