VP choices are much to do about nothing. I don't like the presidential candidate but I love the VP candidate so I am going to switch my vote,... said no ever.
The choice for Veep can only hurt and never help. It is the first presidential choice a candidate makes so it does have some bearing on how the candidate is judged.
Since so few people are undecided voters this cycle, Romney needed to choose a VP that the base liked and that most people believe could assume the presidency at a moments notice. I think Rep. Ryan meets both of these qualifications. I thought Portman was a better choice but I think his time in the Bush administration hurt his chances.