Sous-vide prior to stir-frying...

Hi SallyBR, and welcome!

To your first question, I usually treat sous vide meat going into stirfrys or stews like herbs. I toss them in at the last minute, once it's come off the heat. This prevent over cooking and undoing the benefits of sous vide. So I think once you've seared it, just thinly slice it like you mentioned and then toss it into the stir fry right before you serve it. You can always pour the juices from the bag into the stir fry ahead of time if you want for added flavor.

For the marinade, it depends how long you are cooking it, whether or not to marinade it ahead of time or as it is cooking. Marinades don't work as effectively on cooked food, which will happen in 30 to 60 minutes for a flat iron steak, so you won't get as much penetration. There's really no downside to having a marinade or sauce in the bag (assuming it's not super salty or super acidic) as long as you dry off the meat real well before searing it.

I hope this helps some! Thanks and happy cooking!
Jason

I think marinating it first, and/or cooking it in the marinate

Thank you very much...

it all makes perfect sense, I will give it a try this weekend, have a nice flatiron steak in the fridge winking at me...

Worked like a charm! really liked the texture of the final dish. I added some components of the marinade to the bag, cooked 12 hours. Dried it well, reserved maybe one tablespoon of the liquid accumulated to incorporate in the sauce, did the stir frying with the rest of the ingredients, meat joining at the very end.

superb!

Glad to hear it!