As you mention yourself in your question, he was quoted once as saying his favourite food is steak. Despite the many vegetarian / vegan lifestyle web sites that claim he is a vegetarian, I see no hard evidence of this.
Rather, there seems to be evidence to the contrary. Here he is enjoying BBQ with his cast mates during the filming of The Original Series:
Also, here is an animated version of Nimoy (which he provided the voice for) in The Simpsons enjoying a hot dog:
I suspect that, if he felt strongly about vegetarianism, he would have objected to this portrayal of himself as a meat eater.