Hey dtmp, tracing application performances can be quite sophisticated and the answer to your question is not as simple as it may look. On a 1GB VPS you may be able to run dozens of WP sites well or one badly, and unfortunately it depends on too many factors.
I find top is too crude as a tracing tool, most of the times it won't give you enough details to understand which of your WP sites is being slow and why. That's why there are companies like newrelic and other offering tools to optimize php code. They're not super-cheap, though so before you get there you may want to try other approaches (I have no relationship with newrelic https://newrelic.com/php)
I would move your WP sites from shared to VPS one at the time, with a couple of weeks between changes (at least, depending on the amount of traffic they get) and monitor the server, CPU, memory consumption and disk IO as it changes. You'll have a baseline of resource usage every 2 weeks and you may be able to spot this way the WP installations that is more demanding than others.
hope this helps