Because top lane Vi is infinitely inferior to jungle Vi, that and she doesn't do well vs meta top laners + she's incredibly easy to gank, because she has to use her only means of escape to put pressure on her lane opponent. That being said, you can play her top lane if you'd like, probably using a bruiser build, something like this :
True. I once had a guide for AP Varus and top lane Graves. It didnt matter to me that they weren't viable. But honestly, a lot of Vi's volitile potential is wasted top lane. This is the most likely reason she doesnt have a guide for top lane. It isn't fun. The surprise "HERE COMES THE ****ING TRAIN" ganks from the jungler are, on the other hand, very fun.
You know, viability ( Viability hehe) shouldn't even be a factor. There is no guide because nobody bothered doing one, as Latest Legend said. There are probably guides here on champions that are 10 times less viable in the top lane, but that doesn't matter because someone took the time to actually do a guide about it.
Because Hecarim and Maokai don't simultaneously push the lane with their pressure/escape abilities. They are also much harder to telegraph (Hecarim just kinda runs into you, and Maokai's root is targeted) than Vault Breaker, which makes Vi's trades potentially worse.
Simply put, those champions are reliable and don't overextend as hard.
Should also be mentioned that Vi has serious mana problems when attempting to constantly trade.
And why this is diferent from Maokai or Hecarim, for example. And yet still people doing this champs top lane. And also, why "using his only form of escape to put presure" is diferent from Fiora or Riven, for example. And that are comom picks on top lane?
Simply put, those champions are reliable and don't overextend as hard.
Should also be mentioned that
=P