So this is the overhead. Now each vserver will do something useful. Run apache or run mysql for example. Running a task inside a vserver uses the same resources as running it outside (a vserver).
Memory wise, because of the unification, most task will be sharing the text (program code), so this is fairly efficient.
Now you may want to run very specialized vservers, potentially running a single task without cron and syslog. So goes down the overhead.
For sure it also depends on the activity of the services. The real issue is probably there. If you run 50 vservers each running apache and taking enough hit, you may have performance problem.
Anyway, you will have to try. All I can say is that vserver do not use resource by itself. It only depends on the apps you are running inside and they are using the same resources inside or outside a vserver.
PS: If you run cron on redhat distro, before of task like updatedb. With 10 vservers they will all wake up at 4 in the morning. The load will go up. You may want to disable this.