Why would aVauxhall vectra engine be burning oil?

The engine oil is kept separate from the combustion chamber by the valve seals and piston rings, either of these may be worn out.

These are seals placed on moving parts so the more miles the car has on it the looser they become, several things like letting your oil level fall too low, or operating at too high rpm can cause premature wear of these seals.

In some instances a faulty pcv valve can allow oil to enter the intake manifold when crankcase pressure is too high so check it out as well.