I confess I'm not familiar with the term entrecôte, but looking it up, its pretty clear this is a already-tender cut. So you don't need to tenderize it.
In fact, you probably don't want to tenderize it—at some point, it'll go from tender to mushy. Unfortunately, from frozen, at least according to the Baldwin tables, you'd need over five hours. If you additionally want it pasteurized, that'd probably be over six hours, but he doesn't have a table for that. Either of those times is risking mushy, at least on the outside.
You can reduce the heating time substantially by thawing it in the fridge. This also gives you the opportunity to sear before bagging (killing any surface bacteria, and also improving flavor). You're probably looking at around 3–3½ hours this way (maybe up to 4 for pasteurization, depending on exact thickness).
Finally, once its thawed, you could split it in half, giving you two, 25mm steaks (split before searing, of course). Then you're looking at around an hour in the water bath, or two for pasteurization.
Even with a pre-sear, you'll want to sear after cooking as normal, too.
Your final option is to up the water-bath temperature a bit, and pull before the steak reaches thermal equilibrium. Basically, the last few degrees take forever—heat transfer is proportional to the difference in temperatures—so if you're OK with a doneness-gradient, as you'd find in traditionally-cooked steaks though not as extreme, you can do this. Problem is, I don't have a table for this, so I'm not sure what the time would be at, say, 59°C. You could attempt to rig it with a probe thermometer to find out, but beware cheap probes are not waterproofed where the wire enters, and will be break if water gets there. There are i-Device apps that will calculate these times for you (PolySci Sous Vide Toolbox, Sous Vide Dash) but I haven't used either (no Android versions...) [This is called the deltaT method]
Nope. Haha :)
Brining will give you a softer texture. I have brined duck for varying times (depending on how much of a rush I was in) and brining really does help with the texture.
You can sear it beforehand, some Maillard reaction will help in flavour development, but when I cook sous vide for more than 8 hours, I don't bother: the meat will go green/grey, which means I need to sear afterwards anyway. So I usually just sear at the end.
If your meat doesn't have that much fat, or anything to keep it moist, it can go dry. Adding some butter, or maybe rendered Wagyu fat in the case of your steak, might help.
As for bacteria, the longer you cook sous vide, the more you kill off the nasties.
It so happens that Douglas Baldwin has a short bit about flat iron steak: http://www.douglasbaldwin.com/sous-vide.html#Beef
Hope that helps :)
Best Answer
Carrot and potato can work well using sous vide. In fact, sous vide carrots are delicious. Cook at 90C. The issue will be timing, because it depends on the size of your veg. You are just going to have to check them until you achieve the result you like. Too long, and they will get mushy. For me, a whole carrot usually takes 45 min to an hour. So, a diced carrot could take substantially less time. The same holds true for potatoes...thin slices can take less than 30 minutes, for example.
I have not done them myself, but I assume parsnip and celery root would behave similarly. Use the same temperature, but time will have to be assessed as you go.
Additionally, all of these vegetables will float given that they contain a lot of air. Vacuum sealing will help, but may not keep them under water, as air is expelled during cooking. You will need to weigh down the bags or keep them submerged by placing something on top of them.
In the end, cooking them traditionally might just be the better way to go.