The choice of hosting mostly can be rather tricky. If you use VDS, there may be the following reasons for transition to the next stage, i.e. to the dedicated server.
1. The number of users and the load on server are higher than needed and VDS may not cope with its tasks under such circumstances. Usually it is very easy to determine - the server can just turn off. Pay attention to the fact that for evaluation the number of simultaneous visits should be used, and not the total visits per 24 hours.
2. If you need to tune up your project thoroughly, VDS could not have such abilities. In case of the dedicated server you will be able to configure your software the way you need.
3. In case your project is always growing and constantly developing, it is natural that soon you may need additional options, such as IP-addresses, domains, work with high-loaded databases, working with several websites. Altogether it may be obtained from the use of the dedicated server as your hosting platform.