The concept that a "crust" on meat is protection for moisture loss is a myth. So, searing meat before roasting or braising is not a step to prevent drying. You may, however, want to sear to develop color and flavor. Depending on your recipe, this can be an important step.
When using sous vide, you have three options. A sear before the sous vide step, which can be especially helpful if you intend to do a very long, low temperature cook, as this removes any surface bacteria that might "bloom". Second, a sear after the sous vide step is almost always indicated, as that final crust formation is often desirable. Finally, a sear before the sous vide AND a sear after the sous vide. This allows you to more quickly achieve the final crust, reducing the potential to over-cook the protein you just carefully cooked at to a precise temperature.