It's very much depend on how big and how organized your current structure like? Also what target market you would like to go into?
You can do alot with VPSes like Advance Shared Hosting, where you can categorize your hosting customers into different VPSes for resource management purposes. Also to reduce downtime, let say if you have 500 seperated into 10 different VPSes, if 1 of the VPS down, the rest of your customers in other VPSes are still up and running.
You could offer just emphty VPSes to the market or VPSes loaded with control panel. Customers that want to have root access while dedicated is way too expesive for them, then VPS is their choice. Customers can also start with cheaper VPS plan and upgrade according to their requirement as time passes by.
If you have a higher end hardware, you can even offer server consolidation plan by putting in the DNS, Web, Mail, DB, E-Commerce, ERP, etc.. servers into VPSes.
So, depends on you.