I'm going to assume your using the right threat rotations and keeping HS up since you say you "do everything properly". I'll comment on Vezax, since you mention him specifically in your post.
Do not regem, unless you plan on trying to raise your stam more. Your gear isn't bad, but try to hit def cap/expertise cap through gear choice and not by enchanting/gemming for it. Personally, I have stam enchant on my chest/shield and pvp enchant on my shoulders because Vezax hits like a truck and stam is king on that fight. If you're gemming/enchanting to hit expertise and def cap, then the gear you're wearing is wasting stat itemization on something less valuable (see block rating below). In Naxx, you could afford to gem for def, expertise, hit, etc... in Ulduar, stam is king.
Is it that your TPS is low? Or is it that DPS TPS is high?
Vezax is a tough threat fight to begin with and you need as much stam as possible for it. If it's the only fight you're really having trouble with, then you're probably not doing anything wrong. DPS needs to REALLY watch their threat when standing in shadow pools, ESPECIALLY in the beginning of the fight. With a 100% magic and casting speed buff, they can easily out threat you.
To start the fight, pop Recklessness and quickly go defensive stance to charge in. Hit Shield Slam to get your glyph buff, then hit Shield Block right before your NEXT Shield Slam to maximize it's damage and threat. Your first few hits will all crit from Recklessness and help with initial burst agro. Make sure your casters don't abuse shadow pools until you have a good hold on him. Have someone else watching for Searing Flames interrupts and you just be their backup. That way you can concentrate more on TPS, and not worry about interrupts. (and that person is also taking care of it when you're kiting)
As far as spec goes, you should consider a 15/5/51 or 15/3/53 for more threat (I use 15/3/53, personally. 2min SW's help on that fight too). Impale and Deep Wounds will help a lot. Personally, Block rating is a terrible stat and you have a couple items that have a lot of it, on top of being 4/5 and 5/5 Shield Specialization on your specs. Block value on the other hand is awesome and Glyph of Blocking should be on both your specs, not just one.
This is the spec I tank him with, with Blocking and Shield Wall glyphs (my 3rd glyph doesn't really apply to this fight, but if you want a TPS one, I'd go with Revenge for free Heroic Strikes. Don't use Devastate since after 5 sunders, it does nothing for your threat):
http://www.wowarmory.com/talent-calc...12521330113321
Hope something (anything) in my ramblings helps and good luck with the fight.
-jak