I think he should definitely pick her as Vice President because they have similar views on the world. Their politic is similar and i personaly think they would make a great team. Elizabeth and Bernie know each other for a long time. It doesnt matter if Elizabeth is old, she is verry wise.
Elizabeth Warren would make a logical choice as Bernie Sanders' Vice Presidential running mate. Her penchant for far left leaning political stances melds well with Sanders' even more left leaning views. Both appeal to political populism, and the addition of Warren would certainly strengthen Sanders' political ambitions and increase his chances of becoming President.
Why would you take an effective, progressive Senator and cobble them with the Vice Presidency? This would be a dream for Republicans. Take her away from where she can be of most use and effectively neuter her. Leave her where she is. She is of MUCH more use in the Senate than as VP. Or... Nominate her for the Supreme Court. In those two bodies she can be the most effective.
Should Sanders receive the Democratic nomination to be president of the United States, he should not pick Elizabeth Warren to be the Vice President. Traditionally, the presidential nominee chooses someone to balance the ticket. This includes someone with a different political philosophy, experience, or geography. In this case, Warren comes from the same region, New England, and holds similar liberal views.
We don't need two of the most extreme leftists on the same ticket. I think picking Warren would be a bad choice. I think it would be very polarizing. I think Sanders needs to pick someone more mainstream than he is. I don't think picking Warren would add anything to his campaign.