I'd say your in the green. As a supp you have to use the right judgement when deciding when to take a kill and when not to take a kill. For instance if its clear as day that your adc will get the kill and the enemy barely has a chance of being saved or escaping give the kill to the adc. If you believe there is a good chance that the enemy will escape somehow or your adc's life may be in danger if you don't take the kill than by all means take the kill.
In this situation I'd say you are heavily justified, the vayne is under turret, that's free game territory, there was a chance, and you took that chance because in the end no one can predict the future and it was necessary. For all we know your adc could've missed their skill shot and/or vayne could've backed out and recalled and than neither of you would get the kill gold.
All in all you're fine, your adc was just being unreasonable about it, and if you as a supp get the kill not too big of a deal (especially in this scenario) just try your best to remember "If it is most likely that the adc will get the kill because the enemy has barely any chance of avoiding death than give the kill to the adc". The reason why is because they are the "attack damage carry" you depend on them to kill enemies as much as your adc depends on you to set up the kills for them and keep them safe so that they can get the gold they need to become stronger.
As for personal opinion in my eyes a dead enemy is always a good thing. So if you go and accidentally kill an enemy just remember, "Well at least they're dead". On top of that you get gold that will make you more efficient as a supp and it puts enemies out of their lane so really you getting the kill can rarely end badly if that is even possible.