How to use less Virtual Memory?

Hi,

On my VPS my Virtual Memory is almost 90% / 95% used all the time. I could increase it but the fact is that it's only a recent problem so I'd prefer try to understand how to use less Virtual Memory.

If I'm correct, it could be due to some plugins. How can I find what plugins cause that? The only solution is to deactivate all plugins and reactivate them one by one?
As I've got several sites on this VPS I'm not even sure what site causes the issue.
How can I know which site does?

Well, if you have any direction to help me...

Thanks!