What do you mean when you say "vMotion will not receive the right amount of bandwidth".

When you define shares, you are telling vSphere which traffic get priority / majority of the network link during congestion. That doesn't mean that the services with less shares will fail to operate, they will just be limited in the amount of bandwidth that service has available. How often is your environment doing vMotion and coming under network contention for this to be an issue?

A good read - http://frankdenneman.nl/2013/01/17/a-primer-on-network-io-control/

Cheers, Matt.