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.

The temperature will be climbing all the time, until it reaches about 110 Celsius. This is when you should stop.

The procedure of making candy is to start with sugar syrup and then boil out some of the water, getting a solution which is fully saturated at its boiling temperature (which is above that of pure water) and becomes supersaturated when it cools down. This is how it works when candying fruit too.

So the word "simmer" is technically wrong in that recipe, as a simmer is usually around 85 Celsius. What they are trying to tell you is not to turn the heat all the way up and remove as soon as the solution reaches its final temperature, but to wait some time for the fruit to infuse in the hot sugar water (some recipes even involve repeated heating and cooling over a day or two before doing the final concentrating boil).

I think that it should be possible to do it sous vide, but you'll have to experiment with it. Hold it for longer time at a lower temperature, and for finishing, do the actual candying on a stovetop. The holding temperature will have to be above 70 Celsius so it will break down the cell walls and let the syrup penetrate the fruit. In fact, you may want to blanch it first, then leave in the bath for hours, and candy in the end.

And be aware, once you are on stovetop: the solution will take a long time to start getting over 100 at all, then also a long time to go from 100 to 101, but once it gets going, don't turn your back on it, it only needs seconds to gain another degree! Leave the thermometer in there all the time and remove as soon as it reaches your desired end temperature.

As for the exact desired end temperature, I've seen recipes calling for anywhere between 105 and 115. I don't know if it depends on the fruit type and/or thickness. Again, you'll have to experiment to find out which works out best for you.