I don't believe this information is published however from a Skyline talk at a recent vMUG I was informed that it was "very little" - It's sending encrypted configuration and usage data to VMware in the US but no logging at this time so it won't be a large amount of bandwidth required.
I would install and inspect the bandwidth for 1 vCenter Server and multiply out from there
More information: VMware Knowledge Base
That's very hard to say, it depends on the size of the inventory. Generally speaking the first collection from a vCenter is a relatively big one, for instance, it may be something at around 100MB for a VC with a few thousand VM-s, but after that only changes to that inventory are collected every 2 minutes and those tend to be pretty small, probably a few KBs.