I dinged 90 about 3 weeks ago, and I have enough valor (3000) for my first piece. All my items are blue 463, except for some 476 boots & a neck from LFR. I also have a Relic of Xuen. I upgraded most of my stuff 471 for now.
I'm wondering; what's worth to upgrade/buy first? Should I upgrade my xuen, buy the chest, pants, shoulders, etc.
I will begin raiding vaults/HoF/Terrace next week (my guild already clears them).
After Xuen, any specific piece (chest, shoulder, etc) is harder to get than the others? I know I have a ring & a neck coming from rep factions
After you upgrade your relic of xuen you should have 1500 points then 2500 next week. I'd hold off on purchasing anything or upgrading anything until the week after next week if you are doing any sort of raiding.
I'd upgrade that trinket like others have said so you're no longer capped, then save the remaining 1500 VP til you get Taoren, the Soul Burner from Lei Shi, since that will be your best thing to upgrade.
I don't know if your guild is raiding heroics, or planning on it, but holding off for BiS (for you) pieces is generally the rule. If you're a normal mode guild, it's highly likely you'll get a few of those pieces this week.
If you're going ot be a 4-6/16H guild, some contenders are the bracers off H-Stone Guard (BiS barring the 509 bracers off the BMAH), the cloak off H-Garalon (BiS), the neck off H-Blade Lord (BiS), and of course, Bottle off H-Elegon. Those are all heroics I would expect "casual heroic progression" guilds to eventually have down.
BTW, there's a nice site http://www.wowupgrade.com
Shows dps bonus from upgrading each of your items (armory import), sorted greatest to least. It calculates dps based on stat weights. Not that accurate as simming in simcraft or femaledwarf, but pretty close.