The vinegar was mixed with gall, which is a type of drug/poison. I find it interesting that Scripture says:
"When Jesus therefore had received the vinegar, he said, It is finished: and he bowed his head, and gave up the ghost."
After He drank the gall-laced vinegar, He died. So, it makes sense to me that it was poisonous. Not saying that is proof, but it is logical.