I don't think you have to worry about the chip getting too hot unless it turns out there is a serious manufacturing defect. This chip is quite unlike the ones in earlier MacBooks and is closer in efficiency and thermal performance to a chip you'd find in a phone or tablet. So it'll just throttle down for a bit, not fry, no matter what you throw at it.
Plus, once you have a chip that efficient, it simplifies your heat dissipation... a lot of times you'll have people put chips that really aren't made for laptops in because they are relying on the air flow to cool it to compensate, but air flow can be very unreliable. And that is the exact situation you have with some of the more notorious heat problems of other laptops. If however you have a good chip, and know it can't exceed X heat generation, and its low enough you can passively cool it, that's actually much more reliable... there's nothing mysterious about how much heat can be transferred from a chip to the piece of metal touching it, and the rate at which it transfers (note sturdiness and looks is not the only reason the case is metal). So in a lot of ways this device is actually far more thermally dependable than most actively cooled laptops, even though it'd seem to be the opposite at first glance.
As an aside, have you tried Win 8 for your VM? If you can deal with the UI, Win 8 is about 20% more efficient than 7 due to changes made for tablet support in 8. If you're running 8 in a VM and the VM is on a laptop and you're having heat issues, switching might be particularly beneficial. Also, consider VMWare Fusion or Parallels... I use VirtualBox myself but there's no question that the native Mac versions of the 2 main commercial VMs are going to be more efficient and give you less heat problems on day to day tasks.
