Vilsack is fulfilling the role of "designated survivor" for the Cabinet at tonight's event.
The AP explains:
The administration typically picks one Cabinet member to stay away from the Capitol so the government can continue to function in case a catastrophe were to strike the building.
HuffPost's Jen Bendery reports on what Obama is expected to say in his remarks on Tuesday night.
President Barack Obama will deliver a State of the Union address on Tuesday night built on the message that everyone deserves a fair shot at economic success and everyone must be held accountable for what they do. The next day, he'll begin using that same message as the thrust for his re-election campaign for the next 10 months.
Obama's third State of the Union address, which airs on all major networks at 9 p.m. EST, will map out a vision for boosting the economy and strengthening the middle class, based on four pillars, White House press secretary Jay Carney said in a Monday briefing. Those pillars are American manufacturing, American energy, skills for American workers, and American values. The president also will adopt a tone that ran all through a speech he gave in Kansas last month -- one that many are equating with a historic speech that Teddy Roosevelt gave in 1910 -- that carries a strong message about social and economic equality.
Click here for HuffPost's live blog coverage of the event.