Hi Nick,
what kind of vmdk did you create?
As long as you didn't created an eagerzeroedthick vmdk your tests inside a VM might not show the correct values (depending on the used tool to test).
This is caused by the fact that when your test tries to write to a track which wasn't initialized prior, ESXi will inject a zero out IO in front of your test IO.
This activity is done under the cover, so your test tool running inside a VM isn't aware of that extra IO.
So as your test IO will last longer as he has to wait for the zero out IO, your VM tests will report the wrong throughput and IO response times.
Regards,
Ralf