For an off-tank build, I highly disagree with the way you've set up things here. Any self-respecting off-tank Riven would get valor sooner than last, as it's armor bonus is considerable, especially when paired with some good damage items. Broken Wings should be your last priority as an off-tank, with Ki Shout being your first to go, as stated by Phreak in the demo. Item-wise, I wouldn't choose wriggle for the long-term, and agree with XYZ up there that Phage, and eventually Frozen Mallet, would be a far better choice for an off-tank. If you're building atma's, the extra hp would only help you. Finally, no defensive masteries? An off-tank Riven usually has at least 6 there. Instead, I see you shooting for something that a jungler would shoot for...

Sure you're not making the wrong build? Sorry for my harsh words, but there's a lot of confusion here...

If you're building for survivability, you should look into magic resist and building warmogs before finishing ghostblade. Phage is a great item for Riven if you're looking for survivability and many, though I do not, opt for hexdrinker. Play around with her some more.