The vice presidential debate coverage starts tonight on CNN at 7pm Eastern, and Medicare is bound to come up in conversation.

While the President's health care law has a specific plan for the entitlement, there are still some big questions about Gov. Mitt Romney's Medicare plan.

This morning on "Early Start," Christine Romans looks at the three big questions yet to be answered on Mitt Romney's plan for Medicare if he is elected president.

Governor Romney's Medicare plan is unclear on three issues critical to the 47 million people on Medicare...and the rest of us who may rely on it when we get older.

A reminder: The number of people on Medicare is expected to jump to 62 million by 2020 and up to 90 million by 2050.

The first unclear point of Romney's plan – the cost to seniors. And this only applies to future retirees although he hasn't given a specific age.

We know his plan offers seniors 2 options: A premium support system, or a voucher for each senior to buy private insurance from competing providers, and Traditional Medicare. What we don't know is whether the voucher will be enough to cover the cost of the traditional Medicare option for people who want to stay in it. The Romney campaign has not clarified that, yet.

Second unclear point is the prescription drug plan. Romney says older Americans have nothing to worry about, but there is one big thing that he says will change: Obamacare, or the Affordable Care Act. It saved seniors more than $600 on average last year as part of the government's prescription drug plan. And President Obama says, those savings will disappear under Romney's plan. Romney's campaign has yet to give specifics on how his plan will deal with that increase.

Third, Romney hasn't said if he'll put a cap on Medicare spending. Experts say a cap is needed to promote competition and keep costs increases under control. Plus, bipartisan groups, like the Congressional Budget Office, won't give Romney's plan a score without one. Romney's running mate Rep. Paul Ryan's own budget plan he proposed in Congress, includes a Medicare spending cap. But Romney isn't aligning himself with it.

With tonight's highly anticipated Vice Presidential debate, it will be interesting to see if this issue comes up along with answers to some of these other big questions.