Violet vapors are not given out when X is/are treated with conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$ then X may contain :

Violet vapors are given out when iodide is treated with conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$​.
 

$\mathrm{NaI}+{\mathrm{H}}_2{\mathrm{SO}}_4\to \mathrm{HI}+{\mathrm{NaHSO}}_4$
For Bromide,

${\mathrm{Br}}^{-}/{\mathrm{NO}}_2^{-}\stackrel{\text{ conc. }{\mathrm{H}}_2{\mathrm{SO}}_4}{⟶}{\mathrm{Br}}_2/{\mathrm{NO}}_2$ (both are brown)

${\mathrm{H}}_2{\mathrm{SO}}_4+2\mathrm{HI}\to {\mathrm{I}}_2 (\mathrm{violet} \mathrm{vapours}) +{\mathrm{SO}}_2+2{\mathrm{H}}_2\mathrm{O}$

In the case of Violet, vapors are not given out.