Voting against though is clearly voting for no deal. If that's what they'd prefer, fine - vote against. Just be honest enough to say it.As I said, who is going to mind if Labour don't back a crap deal? It's a Tory project and the Tories need to own it . And when it's a success the Tories can take the credit. And if by some remote chance it isn't, Labour can't then be blamed for supporting it.
Voting against the deal because 'you don't like it' or don't want Brexit even though it's already happened is not a justifiable position, unless there is some legitimate way that a rejection of the deal could lead to some sort of revised deal / extension but that seems highly, highly unlikely.
Who knows, if enough Labour rebel then the Tory no dealers might sit up and start doing the math. That's what happened with May remember.