You might be thinking of his yes votes to limit MFN or elements of it.
http://64.233.167.104/search?q=cache:EW1gu19P5KkJ:www.votesmart.org/voting_category.php%3Fcan_id%3D27110+Bernie+Sanders,+most+favored+nation,+china,+vote&hl=en&ct=clnk&cd=7&gl=us&client=firefox-aVote to adopt a joint resolution that would deny an extension of China's normal trade relations status with the United States.
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Bernie Sanders (I-Vt.): There is a reason why the largest multinational corporations in this country are spending tens of millions of dollars to see this legislation passed, and that reason is they like doing business in China where they can pay people 10 cents an hour, 15 cents an hour, rather than paying the workers in this country a living wage. And there is another reason why the environmental community is opposed to this agreement, why the veterans community is opposed to this agreement, why religious organizations like the National Conference of Catholic Bishops are opposed to this agreement, and that is this agreement is bad for workers, it is bad for human rights, it is bad for the environment, and it is bad for national security. I would hope that the members of this Congress have the courage to stand up to the big money interests who are flooding Congress with contributions, with lobbying efforts, and with advertising, and do the right thing for the vast majority of the American people. Vote against this agreement.