It's not about the build path, it's about how hard it falls off after 1 item, your build becomes so bad for the next 2-3 items and gets outclassed by the traditional build for the rest of the game.
You also can't put a minimum number of times on repositioning with Vayne during her ult, every situation changes.
You'd need a minimum of probably 500 damage from duskblade per ult for it to be as valuable as IE during your ult, that's only at 2 items.
After 2 items that number keeps rising, to unreachable levels.
I'd also like to point out, duskblade is unlikely to be used when her ult is down. You should not be building an entire item that is only effective(not even as effective as ie) for 8-12 seconds every 100~ seconds.
You can keep theory crafting and what not, but I'm telling you now that it's bad.