Most serious illnesses, especially cancer and hyperthyroidism.
Is your cat wormed? This could be one simple reason.
What is Vesti's appetite like? Is it normal, reduced or increased?
This really is one for the vet. But I've had cats who have been seriously ill and always stopped eating and lost masses of weight in the weeks running up to pts (usually Felv related cancer, or, in Max's case, non Felv related malignant tumour). But not all weight loss indicates something fatal.
Have a good chat to your vet. If you have no faith in your vet, go to another.