Anyone can break the Whip, as Jeremy Corbyn did many times. But it is not possible to do so and retain a frontbench position (which the Deputy Leadership, in itself, is not).
Nor does the right to break the Whip necessarily make it right to do so on a particular occasion.
The 1945 Coalition that was the post-War Labour Party finally fell apart when just enough Labour MPs voted to take Britain into the EU to offset the number of Conservative votes against it.
By all accounts, it was never the same again after that. We all know what happened as a result.
But that would pale into insignificance if a military intervention in Syria were to be approved by just enough Labour votes to offset the Conservative votes against it.