Yes Vegetable Frying Oil can be used when mixed in with diesel fuel and a lot of diesel engines today are made to run off vegetable oil either straight vegetable oil or mixed with regular diesel fuel.
You can also mix some vegetable oil in with the diesel fuel for a tractor or other equipment that uses diesel fuel.
Diesel Fuel and Frying Vegetable Oil are compatible with each other and you should have no problem mixing the two together.