Mirnyi is a top doubles player, so he'll be a huge factor on the court. I'm not pretty familiar with mixed doubles, but, in my opinion, it's about who can hold serve more times Victoria or Laura. It can go both ways, really.
Azarenka is a far superior player to Laura, so I think they (AM & LR) are the weaker link. However, the occasion means more than we know so statistics and logic dont always mean at lot, in these cases.
Andy is used to the big stage and the big moments, that will be another little hurdle for Laura to overcome. but with Andy at her side she'll fine. He's taken her under his wing.