Okies. That matchup would be difficult, but not impossible.
Since Jinx will be more powerful than Vayne early game, and roughly just as strong late game, going equal won't be enough. Janna doesn't work well with Vayne IMO, but it is still possible. In this matchup a lot would rest on Vayne's shoulders.
Pre-level 3 or so I would play very defensively and try to farm under tower, Vayne is very good at that. Once that point has passed, trading is possible. Janna needs to stay behind Vayne at all times and attempt to defend her from the other two, her passive will allow Vayne to retreat just as strong as Vayne can play offensively due to Vayne's passive. The trick would be to tumble through leona's e and then counterengage on leona for a single w proc before retreating. Winning those trades is how you'd win that lane.
An early jungle gank would be extremely helpful, you don't even need to kill Jinx, just force her to back before she can attain her BF sword. Since Vayne's bork build path is a lot smoother than Jinx's, if you can force Jinx to get a pickaxe first and Vayne gets cutlass/vamp/longswords, then you can start trading far more often as you can try to force Jinx to back over and over, forcing her to either save money and come to lane at a disadvantage or severely delay her bf sword.
Another possibility would be if they attempt a tower dive at level 6, if Janna can split the two enemies apart then engaging on Leona would be a possibility. Generally throughout the laning phase engaging on Jinx would be difficult due to her long range making it painful, but Leona can be delt with. If Leona takes too much damage to be able to engage, then you'll have the lane advantage.
At level 6 dragon would also be something that may be worth going for, as the area in which you'd be fighting would have more walls for Vayne to condemn Jinx against, and if Vayne can get to Jinx without being locked down, she can kill Jinx. Janna's ult would make positioning the enemy for a condemn easier as well, but the primary point would be that the area around dragon is less linear than the lane, making it more difficult for Jinx to abuse her range advantage due to the number of flanks which would be possible.
I can only really say much about Vayne as I have much more experience with her than with Janna. Generally speaking due to Leona's all-in nature, once an advantage has been attained, then you should be able to keep pressure on the enemy if Leona can't safely engage onto you. If both ADCs have roughly the same amount of gold and experience constantly, then I would say the point where you could fight them is when Vayne gets her bork while Jinx has her IE.
How'd I do? :3