[Enthalpy]

Well, if you have already understood that halogens are fire quenchers, then you're on the way to grasp that mixing a fuel with a halogenated compound will make something not predictable by mixtures laws, as the combustion won't depend only on the fuel's vapour pressure.

There are additional details, for instance mixture laws apply well... in books. When compounds interact, the mixtures tend to have unexpected properties. For instance, when the liquid volumes don't add, the vapour pressures combine unexpectedly as well.

[pgk]

But, fire quenchers and flame retardants are especially designed and aimed to be mixed with fuels and combustibles.
Of course, combustion does not depend only on the fuel's vapor pressure (See above).
And of course, the mixture laws apply well in books, but lead to significant deviations, in practice. Besides, the above proposal leads to approximate estimations only, and not to accurate predictions (why?)