actually, the adapter with VM HW 8 is e1000e, nevertheless, as per my understanding the vNIC speed shown within the OS is irrelevant in the virtual word:
below is the excerpt from Pg 4. of VMware Networking Concepts paper:
"
The speed and duplex settings found in physical
networking are not relevant in the virtual network, because
all the data transfer takes place in the host system’s RAM,
nearly instantaneously and without the possibility of collisions or other signaling-related errors.
"
incase, if you want to limit the traffic for the VM traffic you can use traffic shaping at the portgroup layer OR port ID of vNIC (if using dvSwtich).
http://www.vmware.com/files/pdf/virtual_networking_concepts.pdf