So today I received my tax calculation for Vaud, in which they rejected some deductions which I want to oppose by sending more proofs. But I saw that the letter includes a phrase, which says that it is possible to oppose, but

"La réclamation ne peut mettre en cause ni les éléments imposables ni le calcul de l'impôt"

which means I can't oppose to the taxable elements or the tax calculation.

How should I understand this phrase, should I still write a letter and provide more proofs for my deductions?

Yes, of course, and the authorities will correct your taxation if the deductions are valid and you can provide proof that they apply to you.

That would entirely depend on the original reason for rejecting you claims in the first place....

- was it rejected for a lack of proof
- it is not an allowable expensive
- a point of law
- something else

I mean I will send my letter anyways, nothing to lose. But I'm just trying to understand the meaning of this phrase, which is included in everyone's tax decision, not just my case in particular. My interpretation is it claims that the calculation is final, you can oppose other things but not this?

No, it just means that you can't dispute what is taxed, nor the method of calculation of the tax.