Sanders will probably vote for Tanden. It's clear his goal as a high ranking socdem is to try to get as many decent concessions from the dems and the Biden admin as he can. Although I would absolutely love to see Tanden eat shit, it's probably true that anyone that Biden would appoint will be functionally more or less the same. With that and Bernie's goals in mind, he's not likely to vote against her in the end. At most he, like others have suggested here, will try to use his necessary vote to gain a concession.