Essentially, here’s the criteria I would use to judge things – if your site is made up of primarily static, HTML-based content, then you probably don’t need a VPS package.  However, if you have a large amount of files stored, multiple sites, dynamic content, and the possibility of major traffic from time to time, then you might consider upgrading to a VPS. It’s a powerful package that allows you to do more than you could with a shared hosting plan, but requires less investment than a dedicated server.
Similarly, in a real dedicated server, you will pay for the entire server that is not shared with anyone else. You will get complete control over all services. Unfortunately, it is also the most expensive hosting option and needs some technical expertise to manage. It is commonly used by those who have websites with specific scenarios, most commonly extremely high traffic and tight security requirements.
Yes, it’s secure. VPS security comes from each instance’s isolation from the other environments on the server. Contrast that with shared hosting, where environments are sharing the same resources and can be affected by each other’s vulnerabilities. A denial of service attack on a website in a shared environment can bring down other companies’ data and websites hosted on that server, where each VPS environment is isolated and protected.

The force driving server virtualization is similar to that which led to the development of time-sharing and multiprogramming in the past. Although the resources are still shared, as under the time-sharing model, virtualization provides a higher level of security, dependent on the type of virtualization used, as the individual virtual servers are mostly isolated from each other and may run their own full-fledged operating system which can be independently rebooted as a virtual instance.

Ask most website owners about hosting and they’ll happily explain what shared, dedicated and cloud hosting is. But when it comes to virtual private servers, lots of them aren’t 100% sure what VPS are or what they’re best suited for. Some don’t even know that there is a VPS hosting option. So, to put everyone clearly in the picture, this article will give you a full introduction to VPS.
In answering this question, maybe it’s better to examine how VPS hosting fits in to the overall offerings of most hosting companies. Shared hosting is just that – your site is hosted on a machine with a bunch of other sites, and each of you share the same resources, including RAM, disk space, and CPU.  Your site uses what it needs if it’s available, and if it’s not – well, that’s the limitation of shared hosting. Likewise, a dedicated server is also self-explanatory –  your site is the only one hosted on server, and you have all the aforementioned resources available at your beck and call. Dedicated hosting is therefore  for those large sites with big databases and lots of traffic, whereas the limitations of shared hosting’s usually prevent it from housing that kind of site. Dedicated servers are also relatively expensive, while one can get a shared hosting plan for under $10 per month.