Even if plants might feel pain, we are certain that animals feel pain. Also if you think for whatever reason that plants feel pain, then, well, a vegan diet uses less plants because its a more efficient food source. Plants feeling pain, whether true or false, isn’t an argument against veganism in any way.
the crux of the argument that they might feel pain is not that it is wrong, but that it is inevitable, so it cannot be wrong
when somebody raises the objection that plants feel pain, it’s not an appeal to hypocrisy. it’s a statement of fact whether we can prove it or not. and it’s the premise of a larger argument. that argument goes
pain is an inevitable facet of food production
food production is a moral good
an inevitable facet of food production cannot make food production bad
therefore
food production remains a moral good
your rebuttal was targeted at defending against the accusation of hypocrisy, but the devastating bit has nothing to do with the hypocrisy.