We have seen MANY companies try to push VICIdial as hard as they can, and there is really nothing to stop them because there are no license limits on channels or agents.
When I say "safely", it's all about measures of reliability. When you overload your dialer, then things start to get missed, like a voice packet here or there, or a command isn't executed, or a recording isn't started(or is started several seconds late). The response to commands slows down and there are delays in the database because the dialer can't handle the number of CPU threads it has to maintain. It's not like the dialer just shuts down, although that can happen. The dialer just becomes less reliable and slower, and as a result, not everything that is supposed to happen will happen like it is supposed to. This is why we have recommended safe loads. A rule of thumb is to look at the loadavg on a dialer, and that it shouldn't get above 1 x "the number of CPU cores", so for a quad-core dialer, 4.00 would be a safe loadavg. Although, we have noticed on newer systems running Asterisk 11 that the loadavg can be inflated and not report a true load properly, so in those cases you could reliably load those systems to 8.00 without any issues.
It's all a trial and error process really. You have to find out how much your system can handle by testing it, and the results for the same hardware could be very different for clients that have different requirements.
For example, we have a VICIhost client that routinely places 1.5 million calls per day on their 12-dialer cluster. They are very focused on capacity and reliability, and they follow our recommendations for how to streamline their processes and do everything possible to safely optimize how they system works. We also have another client with more than double that number of dialers, but they do several things that we do not recommend doing, like using MySQL triggers, and complex custom SQL statements, and having over 20 million leads in their database. This other client can have system issues when they try placing just half of the number of daily calls as the first client I mentioned, even though they have double the number of dialers. So, you can see that reliable functional capacity is heavily dependent on a lot more than just the hardware you use.
As to whether I sleep, I like to take naps when I can