If you are not playing Tasbu decks right now, i think you are doing it wrong.
I've been tweeking this deck over the past couple of days and i'm now top 20 in masters, though it might change.
One thing of note when playing it: you really want to have something like 3-4 power in your intial hand, because getting to your powerfull 4 and 5 drops on curve is critical to win games.
Three cards I can think of off the top of my head are Rhysta, Acantha's Herald (fits in slot and helps ensure you get the power you need,) Kerendon Merchant (also fits in slot, but I'll go with your opinion on if the deck wants more access to the market,) and Ripknife Assassin (historically very good in the mirror if this is indeed a T0 deck.)
As for Vishni, Lethrai Highblood it was one of the latest additions for the deck. It is best if shifted for 6 power, but you can still play it on curve for 3 power. The great thing about this card is its flexibility. I've played a lot with Lethrai Direbeast in that slot, a 3/3 lifesteal is no joke and you can give Tasbu lifesteal late game. I would advise against Rhysta cause we are not trying to be a aggro deck, we are trying to win between turns 6-10 and not be able to block is a big liability. I personaly don't like Ripknife Assassin so i try to avoid putting it my decks. Kerendon Merchant might be an option but the one thoughness makes it very fragile to Vara's Favor.