What temperature should "simmer" be for candying fruit?
I don't think you physically can achieve the same results with a sous vide machine. Typically, those are designed to hold a water bath at a specific (relatively low) temperature. You can't hold water above its boiling point in an open container, because it's, well, boiling and will eventually evaporate. (You could do this in a sealed container, which is the principle behind the pressure cooker, but that's very different than sous vide.)

Here's the thing: simmering is gentle boiling, and by definition it occurs at (or just under) the boiling point of water. You cannot raise the temperature higher than that without adjusting other variables like pressure. Because solutes (like sugar) raise the boiling point of a solution, syrups will often come to a simmer well above the boiling point of plain water. So even if you placed your syrup in a bag, placed it in the water bath, and raised the water bath to a boil - the syrup in the bag wouldn't be boiling. You would have to add pressure or another solute (like salt) to the water bath to get the temperature high enough to also boil the syrup, which could very well damage your sous vide heating element.

The "half-candied" recipe you've linked is an interesting one, because it's really not traditionally candied. Instead what they're doing is infusing the orange with a high sugar content in order to preserve it in a similar way. That can only really be done in a low-pressure, vacuum-sealed environment, because it's using the lack of pressure to force the orange to absorb the syrup instead of boiling out the liquid to be replaced with syrup.

tl;dr: The temperature you need for the syrup to boil is higher than the temperature you can achieve in a sous vide bath. You will have to find another way to streamline the process.