It should be noted that, it is not Ventoy to decide to boot in Legacy BIOS mode or UEFI mode.
It is the hardware firmware (BIOS) that decide to boot Ventoy in Legacy BIOS mode or UEFI mode.
You can do something to make Ventoy doesn't support UEFI boot.
After install, there is an EFI directory in the VTOYEFI partition(the small 2nd partition with 32MB size), delete the EFI directory
then Ventoy will be unbootable in UEFI mode.
But you should known that, if you do that it will fail if the BIOS try to boot Ventoy in UEFI mode.
It is the hardware firmware (BIOS) that decide to boot Ventoy in Legacy BIOS mode or UEFI mode.
You can do something to make Ventoy doesn't support UEFI boot.
After install, there is an EFI directory in the VTOYEFI partition(the small 2nd partition with 32MB size), delete the EFI directory
then Ventoy will be unbootable in UEFI mode.
But you should known that, if you do that it will fail if the BIOS try to boot Ventoy in UEFI mode.