The ventoy disk will be bootable (as the MBR must contain the specially modified Ventoy code) and the first two partitions must be present with the 2nd one being the Ventoy boot code partition.
So for MBR booting, I imagine you would just use a grub chainloader command to boot the disk mbr bootcode
maybe
chainloader (hdX)+1
where X is your Ventoy disk
For UEFI booting, you can maybe chainload the Ventoy EFI boot files ?
chainloader (hdX,2)/EFI/BOOT/BOOTX64.EFI
So for MBR booting, I imagine you would just use a grub chainloader command to boot the disk mbr bootcode
maybe
chainloader (hdX)+1
where X is your Ventoy disk
For UEFI booting, you can maybe chainload the Ventoy EFI boot files ?
chainloader (hdX,2)/EFI/BOOT/BOOTX64.EFI