So you have just run your test, of say 220 users, and now you know what your capacity of users are (197). Wouldn't it be great to take that info and determine where the bottleneck is. That's where overlaying external data comes in to play.
First we see our results, it looks like we hit the VSImax at 197 (uncorrected).
Next, lets use external data overlaying so we can see what the CPU was doing on our ESX Server during the test.
Click on File | Import (then choose your ESXTop Capture file).
Once its open, you can filter which counters you would like to see. In my example, I will pick the %Core Util Time and click open.
Now you will see the overlay of the VSImax v4 data and the counter we chose.
Tip: To help you only view the data you want to see, you can modify the scale, by using the Scale icon.
As well as uncheck any of the line graphs you don't want to see.
Now this looks a bit more interesting. We can now see that the CPU (yellow line) was peaking at about 95% when we hit our VSImax of 197. So now you can do a direct correlation between VSImax and what was happening on the server.
This is just an example that shows how you can do this using the ESX Top data. We can also do external data overlays with most 3rd party programs that can output to a .csv file.