When I look to buy new things for my tank set I usually look at the biggest upgrade I can do. So I look at my weakest item and try and upgrade that. Also you have to keep your hit and expertise where they need to be so keep an eye on that, but I usually go for the biggest HP gap that I'm missing.
So usually just get whatever is your weakest item if you can. Also remember things like VOA that drop just hands and legs. Try and be smart about it and see what you can buy from the AH,vendor, and frosties.
Run VOA for your free hands and legs. Because you are a tank, it should be easy for you to get in a group were you can control over who else is in your run, for less competition.
Get the shoulders IMHO, so when you do get a VOA drop, you will have your 2 set bonus.
I Went for the Cloak, then the Stamina trinket, then the t10 shoulders. I had been lucky enough to get the sanctified t10 hand in a voa25. I suspect my next t10 piece will be the head, and bank for t10 legs from voa for a 4 set bonus.