Can I use sous vide instead of pressure cooking

If this is a recipe that will be served immediately, you may be able to use an alternate cooking method. If the reason is to store in your pantry, NO, do not use any method other than pressure cooking. From Pressure cooking Wiki

The standard cooking pressure of 15 psi was determined by the United States Department of Agriculture in 1917. At this pressure, water boils at 121 °C (250 °F).

You need this higher temperature to safely process certain items for storage.

From Fresh Preserving.com

When preserving vegetables, meats, poultry and seafood, safety is key. To keep what your canning safe to eat, and fresh tasting, you’ll use the Pressure Canning method which heats the contents to 240º F eliminating the risk of foodborne bacteria. You should also know that if even you’re mixing high acid foods with low-acid foods you must use the pressure canning method

Assuming, as Debbie notes, that this is not a recipe for preserved food, yes you should be able to, but I'm not sure it is the best/only option available to you. Choosing the substitute method should be based on what your best judgment tells you is the reason the recipe author chose the original method. (BTW, including your recipe in questions like this will help get you better answers faster.)

Most things you cook in a pressure cooker can also be cooked on the stovetop or in the oven as well with modifications on time and temperature, or, yes, adjusted to be cooked sous-vide. (Full disclosure -- I haven't yet tried sous-vide. I do cook a lot with pressure cookers, and frequently adjust and adapt my recipes so I can use my pressure cooker.) I think using a boiling water bath (if you have a large enough pot, such as for home canning) might be closer than sous-vide to the pressure cooker-canning jar method, but sous-vide should probably work, too. If you do want to do sous-vide for this, just use a reliable time/temp guide for the specific veggies in question.

Cooking in a pressure cooker increases the heat above boiling so if the recipe is expecting the duck fat or the vegetables to do something specific at a higher heat then this would not happen in the lower temperatures of the sous-vide method. If you need the higher heat to get the fat to penetrate the vegetables, for example, you might be better off oven-roasting at the same temperature.

I doubt this is the case. I suspect they are using the pressure cooker for decreasing the cooking time, although that depends on which vegetables and how long in the cooker. But use your judgment here -- if a recipe from a reliable source requires you to go through the hassle of setting things up in a canning jar in a pressure cooker for a very short time, then cooking speed IS NOT the reason, and probably sous-vide isn't the right choice as a replacement.

Cooking in a canning jar will prevent browning and caramelization; sous-vide would give the same results in this respect, as would cooking in a canning jar in a boiling water bath.

NOTE: Without having read the recipe, I am guessing that it is likely that the recipe is only partially cooking the vegetables in the pressure cooker and completing the cooking in another way with the rest of the ingredients. In that case, you'll need to adjust your cooking time accordingly for whatever substitute method you use.

Have you considered using the micro-wave instead?

ChefSteps (which are very fond of sous-vide) seems to prefer this.

Quick and Easy Vegetable Confit