To connect Grub with Ventoy I use the Grub Customizer with the entries

insmod part_gpt
insmod ext2
insmod fat
insmod exfat
insmod ntfs
insmod chain
search --no-floppy --set=root --fs-uuid xxxx-xxxx (= UUID of the Ventoy boot drive)
chainloader /EFI/BOOT/grub.efi

oldfred had already linked an integration with grub. This is how the whole thing works in my system.