One of the sony-vaio-z-series launchpad team members has reported the way to successfully switch on/off the nvidia card in VPCZ13 models:
Re: [Sony-vaio-z-series] Ubuntu on VPCZ13*
- Installed sony-laptop-zseries-0.9np7
- Grub load options: "i8042.reset i8042.nomux i8042.nopnp i8042.noloop acpi_osi=\"!Windows 2006\" acpi_backlight=vendor"