**Yes—a sous vide circulator can slow-cook food in a second vessel inside its water bath.** Your bain-marie idea is sound. The main limitations are temperature, heat transfer, and capacity.

For that setup:

- Use a heat-safe inner container, preferably metal, supported so it cannot tip or float.
- Have the bath water reach roughly the food’s level, while keeping it out of the inner container.
- Leave room around the container for water circulation, and respect the circulator’s minimum and maximum water levels.
- Cover the food and, where practical, the bath to reduce evaporation.

**The temperature determines which recipes will work.** The food can only approach the bath temperature, and a large bowl of thick stew will heat more slowly than a thin sous vide bag. A bath around 85–90°C can produce braised meat and cook many vegetables, but ordinary slow-cooker timings may need adjusting. Baldwin describes braised meat textures at 80–85°C; some circulators, such as the Anova Precision Cooker, top out at 92°C. You therefore cannot count on this setup to boil food or reduce a sauce substantially. [Baldwin’s cooking guide](https://douglasbaldwin.com/media/Sous_Vide_Cooking_for_CRC_Handbook.pdf), [Anova specifications](https://anovaculinary.com/products/anova-precision-cooker)

For meat cooked at lower temperatures, use an established sous vide recipe with appropriate heating and pasteurization times. The bath reading does not tell you how quickly the centre of a bowl has heated. [Baldwin’s safety and heating guide](https://douglasbaldwin.com/sous-vide.html)

**Energy use need not be unreasonable.** Heating the extra water takes energy initially; after that, consumption mostly depends on heat escaping through the container and evaporation. A covered, suitably insulated bath helps. The circulator’s advertised wattage is its maximum power, not necessarily its continuous consumption.

For inexpensive equipment, the choices are:

- **If you buy the circulator anyway:** use an existing pot plus an inner bowl—or put the food and cooking liquid directly in a suitable sous vide bag. That is the cheapest additional setup.
- **If you want conventional slow cooking frequently:** compare a basic slow cooker plus a budget circulator against a combined appliance. Two inexpensive devices may cost less and offer more flexibility.
- **If you want one appliance:** choose a model explicitly offering both slow cooking and sous vide. Examples include Hamilton Beach’s Temp Tracker and certain Instant Pot multicookers; check the exact model’s functions. [Hamilton Beach manual](https://useandcares.hamiltonbeach.com/files/840294200.pdf), [Instant Pot example](https://instantpot.com/products/instant-pot-6qt-multi-cooker-with-accessories-target-exclusive)

There is no universal cheapest purchase without local prices, but **your proposed bain-marie is a workable way to get both methods from a circulator**, provided you adapt recipes to its temperature limits.