I would advise you to focus on Vray.
I started on Mental Ray in Softimage 3.5 in 1997. It's a great engine, but it's aging, and fast.
Most of it's inner structure is outdated and it's struggling to meet today's standards.
Nvidia fired most of the dev team at mental image, it's clearly dying a slow, painfull death.
All the companies in France are switching to Vray in Maya for commercials, and my friends in the US tell me the same things. They used to be pro Mental ray people!
Vray is simpler to use, and you get better results faster.
You'll have better chance finding a job if you know Vray now, unless you're aiming at some old company that still uses MR.