Here's a marinade & sauce recipe for flank from one of my favorite I-Net cookers.....
http://www.thesmokering.com/forum/viewtopic.php?t=28190&highlight=flank+steak My experience has been, for cuts like that, some kind of marinade / juicing up proceedure goes a long way toward tenderizing...............a lot of folks use wine of some sort, Woosty sauce is usually good, garlic in powder or fresh, you may be able to use apple cider vinegar for tenderizing, but as I just now thought of it & haven't actually tried it, maybe hold off on that.......
Here's my very 1st internet cookup post, using round steak, but I've done the same with flank many times.......used to lay it out & slice it to about 1/8" thick (get maybe 3 pieces from the average thickness whole ) & roll up all kinds of stuff inside............
http://www.thesmokering.com/forum/viewtopic.php?t=49280&highlight=flank+steak