while you can't prove a negative, it is possible to find evidence for a positive claim. so, very much, you don't know that. the truest thing anyone can say is that there is not a conclusive study that supports the claim.
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.
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.
Pain might be an inevitable facet of food production (crop deaths). But that doesn't mean we shouldn't try to eliminate as much pain as we reasonably can.
Pain is an inevitable facet of surgery
Surgery is a moral good
an inevitable facet of surgery cannot make surgey bad
surgery remains a moral good
The fact that pain is inevitable to surgery doesn't mean we should stop giving patients anesthesia and pain medication.