The relevant cause of action for a corporation is the tort of...

The relevant cause of action for a corporation is the tort of injurious falsehood, rather than defamation. It's a bit harder to prove, but theoretically available to Tyro. But Tyro shouldn't waste its time and energy on crap like that, and nor should you. Viceroy's 'research' is pretty self-evidently weak, but the outage is a genuine issue (albeit one that is now more than factored into the share price).

While we're talking about legal actions, the class actions are pretty hopeless - Tyro's contracts with merchants will exclude consequential loss and make it very clear they're not responsible in contract, and any claim in tort would also be weak. Any shareholder class action looks basically non-existent (no evidence of anything misleading or anything of substance that hasn't been disclosed), and that's why you haven't seen the big players talking about it at all (Slater's, Maurice Blackburn, the big litigation funders). I wouldn't be too concerned by a third-tier firm (ie Bannister) announcing it's looking into things.

(Source: litigation lawyer with experience in class actions and defamation)