I'm sure the Reds have talked about, if not with Votto's agent, at least internally. He's under their control for 3 more years due to arbitration, and he'll get continually more expensive.

When do you seriously begin discussing an extension that would give him some security in case of a major injury while buying out his first 2 or so years of free agency?

No need to do it right now. But, if by the all star break he's still putting up his usual great numbers I try and sign him for about 5 years. Beyond that it's certainly debatable whether the Reds would be able to afford him and keep the rest of the team strong. But, I'd let some other team give him the huge, huge deal he'd then receive at about 31.