Panel 1: Between the snooty expression and extended pinky, Jon's body language while reading food labels to Garfield is one of a man trying to placate his friend's unhappiness with reality by gussying up the ugly truth. This is even funnier because the circumstance is his cat's dislike of gross canned food.
Also: is Jon reading from an unopened can? It is not outside the realm of possibility that there's a second can of the same food, but why confuse the issue?
Panel 2: That's a "pie"?
Panel 3: Garfield's sudden enthusiasm for the meal has nothing to do with how good vulture may taste, but in a scavenger becoming the scavenged. Garfield is motivated by A) the sense of satisfaction and purpose we derive from seeing karmic justice dispensed, and B) the sense of power derived from participating in the same. There's an uneasy tinge of sadism in the scenario for those in the witness stand, but stranger still is our hero's personal vindication. What have vultures ever done to Garfield? Picked over his corpse and disgorged strips of his fatty orange flesh into the open mouths of their young? Certainly not. Perhaps it is the pride of a hunter, in this case the domestic cat, who resents the parasitic air that scavengers have been shouldered with in human anthropomorphic thinking. I doubt it, since Garfield does not look down on thievery, underhandedness or laziness. Instead, he just enjoys exerting power over a creature that has been weakened, sapped of challenge and ground into a brown paste. Garfield is a bully, even when taking unmotivated revenge on a bowl of reeking canned sludge.