I was speaking with a customer recently about networking when the topic turned to the virtual nic or vnic bandwidth available between a VM and the virtual switch. The customer complained that the server guys were performing "gymnastics" in trying to change the speed from its reported value of 100Mbps to a more palatable speed of 1Gbps.
Fortunately, one of our senior engineers was present at the meeting and was able to explain (more completely than me) that the bandwidth *reported* by the guest VM on the vnic interface is a function of the NIC driver on the guest VM (e.g. e1000, vlance, etc). The actual vnic is implemented in software, so is only limited by the processing capabilities of the host itself.
So, ...ignore what the guest reports as the NIC link speed ...(and bad on us for not making this more apparent!)
p.s. if you *do* want to set a limit on the traffic produced by a VM, you can use the "TX Rate Limiting" feature in the Port Group definition. Here you can set the maximum and average bandwidth as well as a "burst" size.