The side rail on the Vienna crib looks very similar to my Baby Cache Montana crib. The Fisher Price rainforest mobile fit my crib well.
When are you planning to transition baby to the crib? Most mobiles become useless once the baby starts to push up on hands and knees and on my Fisher Price mobile, it says to not use the "arm" part once the baby is 5 months. We transitioned DS to his crib at 5 months and only used the "arm" part of the mobile for maybe a month. After that, we had to remove the "arm" and just have the music/lights portion on. Some mobiles don't allow this detachable option though so if are planning to transition baby to crib later on, you might want to skip the mobile completely.