Vahlok was one of the most powerful dragon priests (being the ruler of Solstheim and all) so where is his mask? Could it be possible that Miraak stole and warped his mask during the fight or possibly when he returned to Solstheim as a sort of revenge?

I dunno, it just seems odd to me that the second most powerful (known) dragon priest was not granted a mask.

I doubt Miraak stole the mask. Miraak was probably given his by Mora.

Vahlok's could still be locked away in an undiscovered tomb somewhere for all we know.

Why wouldn't he be buried with his mask while all the other dragon priests were?
The only other explanation i could think of is that the mask Konahrik belonged to Vahlok and that he couldn't have worn it due to it already being in skyrim. the only problem with this theory is that the mask always shares the name with the dragon priest who wields it.
If Miraak stole the mask, he could then warp it using Mora's power, causing it to change it's shape.