Don't slice the meat.
Put a large water pan with hot water in the bottom of the electric smoker and set it to 140 or maybe a tad higher, just ensure the water doesn't get much above 130. (air doesn't conduct heat as well)
Place the tenderloin on a rack above the water and hold for up to 4 hours. (personally i'd say much longer but the food police will object)
Remove and slice to order - use a torch to crisp the outside after cutting by placing on an all metal cake stand then rotating the slice while torching the outside. (makes it nice and crispy but still MR inside)
Plate on warm plates over mashed or whatever you are using to hold the heat as the slices will cool off fast although with a full 12oz you should have quite a bit of time to play with. (it is on the generous side)
--- alternately ---
You can do the water bath by using the beer cooler hack and holding the large mass of water at 130f so you don't cook it past Med-Rare. No need to vacuum pack just use good zip-top bags and squeeze all the air out. Use the drain on the base of the cooler to remove water as it cools and add fresh hot water as required to keep the temp stable.
I'd keep the roasts whole and slice as needed but with a 16oz portion there shouldn't be any problem holding individual servings, just don't let the temp creep up as the hot water transfers heat really fast and even a short spike to 140 will make medium cooked portions. This also works to your advantage as if someone want well-done just drop it in a pot of 160f water for a minute or two and you're done.
With such a large volume of water a beer cooler is actually very stable if you keep the lid on it.