Which vendors can upgrade my gear?

I believe any vendor can do this, but it doesn't work with all weapons and armor. Some items like Dragon Armor have a set level, Dragon Armor's being tier 6. Other items have a level range that changes based on the level you are when you get the item. Warden Commander Armor, which has a level range of 3-7 is a great example of this.

So if you are around level 20 and your Warden Commander Armor is Tier 4, you can simple sell it to a vendor leave the zone, go to party camp or something, then head back to the vendor and buy the now Tier 7 Warden Commander back from him.

You will lose money doing this and I'm not sure how many items it will work on. So if you're going to play around with it you should always save before hand.

I should also note that this will only change the base damage on weapons and the base armor on armor. If a chest has a magical bonus of +1dex at tier 1 it will still only have a +1 dex bonus at tier 7. The only thing that changes is the armor on it, as well as the str it takes to put it on.