Using VLOOKUP to compare data on two different worksheets

Hi, I'm hoping someone can help, I have very little experience of using Excel, and being faced with a large amount of data which needs to be organised by the end of today, I am panicking slightly.
I have attached an example of the type of data layout I am currently dealing with, I cannot upload the actual files because they are work files, but the one I have uploaded is a close approximation of what I am working with, albeit with a dozen rows and columns rather than the few thousand which exist on the actual file.
Sheet One contains data imported from an up-to-date online database. This one is correct and the data on Sheet Two should be measured against it. Sheet Two only contains data imported from the CMR program, although it is correct and up to date. What I need to do is to import the data on Sheet One in to the CMR program, obviously minus the entries which already exist on Sheet 2.

The example I have created (and attached) contains four fictional company details. So, I would need to retain the ones on Sheet One which do not exist on Sheet Two and remove the ones which do.

I understand that the VLOOKUP function is the best function to use to do this, but my question is: how?