Can I marinade flank steak that I'm planning to sous vide?

The best I can tell you is based on my own marinade and experience. I seal my flank or skirt steak up in a vacuum bag and let it chill for a few hours to get the flavors into the meat. Generally that is best done for at least 2 hours, and up to overnight. I do sous vide my meat in the marinade, 135F for about 2 hours, and then I finish it on the grill. It's always worked out quite well for me that way. I also like using a dry rub instead of a wet marinade from time to time, and letting that sit for at least overnight- vacuum sealed, and then cooking sous vide and grilling. I actually think the dry rub tastes better, so that's something else to consider.