Well the evidence that we have to base our decision upon is the studies you have read. I have read them too. What I think we can garner from them is that a certain degree of exogenous creatine benefits cognition. Vegetarians because they don't eat meat (this should be our definition of vegetarian for these purposes) benefit from a bit of creatine, because they have lower levels than meat-eaters. A meat eater in their youth doesn't benefit from creatine supplementation in that way most likely because they already have sufficient exogenous creatine from the meat that they eat. It is a matter of creatine status in the body in my view. I don't have precise figures but I would assume that any significant period of time without exogenous creatine lowers levels and thus causes one to lose the benefit that one previously had from higher levels. That is why the meat-eaters don't benefit from the supplement, they already have the benefit of the nutrient in their diet. So if you have been avoiding meat for months, maybe just weeks, I'm not entirely sure, you will benefit from either meat or creatine supplements, but if you have been eating meat for a significant amount of time then you don't need the creatine for those specific purposes, you already have enough and are already enjoying your cognitive benefit of exogenous creatine.
Whew, that's about as good an answer as I have. But the simplest answer is that exogenous creatine confers a benefit whether is be from food or creatine, but you only need so much. Having been a vegetarian for many years is less likely to matter than the amount of creatine you're getting now. My assumption is that creatine levels in the body matter to the ability to produce phosphocreatine and thus product ATP in your brain faster. Because long-term vegetarians benefit from creatine, we can pretty much deduce that there is nothing about being a long-term vegetarian that changes the benefits of a moderate intake of creatine from the diet, vegetarians only benefit from the supplement because they're lacking the nutrient already whereas meat-eaters aren't.