Why was a vote taken in the middle of Old Major's speech? What was the distinction being argued about?
The vote is taken regarding rats to settle the distinction between their status as the animals' comrades or their enemies.
After Old Major gives his reasoning behind the need for the animals to rebel, and before he tells about his dream, there is an interruption requiring a vote: some rats had appeared and were promptly chased by the dogs, so it's apparent that the animals need to decide whether to include wild animals in their newly established union.
At this moment there was a tremendous uproar. While Major was speaking four large rats had crept out of their holes and were sitting on their hindquarters, listening to him. The dogs had suddenly caught sight of them, and it was only by a swift dash for their holes that the rats saved their lives. Major raised his trotter for silence.
‘Comrades,’ he said, ‘here is a point that must be settled. The wild creatures, such as rats and rabbits — are they our friends or our enemies? Let us put it to the vote. I propose this question to the meeting: Are rats comrades?’
Keep in mind that up until this point, Old Major had spoken as if all the animals needed to consider each other "comrades," like close friends or brothers, in order to rise up against the tyranny of humans. But rats and rabbits are animals, too, even though they're not actually residents of the farm: so is it okay for the dogs to chase and kill the rats, or should the animals treat those rats as friends? That's the distinction being argued.
In the vote that takes place immediately, "an overwhelming majority" of the farm animals believe they should, in fact, treat rats and rabbits and other wild creatures as friends rather than enemies. This leads Old Major to state authoritatively that "Whatever goes upon two legs is an enemy. Whatever goes upon four legs, or has wings, is a friend."
This episode should leave you with something to think about. What could be foreshadowed by the dogs' sudden chasing of the rats? Probably division within the ranks. And, since the dogs were outvoted, does it mean they will go along with the new decree to treat rats as friends, or will they just continue to hunt them? Has order really been established or just the appearance of order? To what extent will these votes and agreements really control some of the animals' behavior, or for how long? As you can see, the issue of the rats foreshadows trouble for the animals' new alliance.