So reading https://www.dreamhost.com/hosting/wordpress/
Here are the differences between Wordpress Managed Hosting and the VPS Plan at Dreamhost
Wordpress Managed Hosting is 20.00 per month per site. You can only have one Wordpress site per Wordpress Managed Hosting. Dreamhost will maintain the server, keeping it secure and up to date, upgrade your WordPress installation automatically for you, and backup all of your files on a regular basis.
If you have multiple Wordpress Installations, you would be spending 20.00 per each one, each month.
Now, VPS is an entirely different beast. You are responsible for keeping it secure, upgrading WordPress, etc. Now, by default, securing it is partially handled, as Dreamhost uses a lot of sane defaults. They also make it dead simple to add new sites, new wordpress installations, etc through their control panel. So you don’t have to worry about learning Linux right nout of the box. You will want to eventually become comfortable with SSH and login to your VPS to upgrade the kernel and other software to keep it up to date, but since they use Ubuntu, the process is very simple.
You won’t have to setup Apache by yourself, that is done automatically via their control panel, you will have full access to their support, you can manage users, one click installations of Wordpress, setup of the databases, all through their control panel. Same with email setup, mailing lists, billing, et al. Backups are done automatically too. But you are encouraged to also setup a backup plan of your own choosing which requires more hands on/Linux savvy approaches.
If you want to upgrade your VPS you can expand both memory and CPU through their control panel, if you want to upgrade to Ubuntu 14, you will have to do that yourself (you may be able to ask the Support team to do it, but I’m not sure how that process would go, or if they would require some sort of fee for it).
Either way, the VPS is likely what you want, unless you want to spend 20.00 per site that you plan to transfer. I strongly recommend talking with their Sales Chat first to ensure they still have a 90 day guarantee. I think they do. And I would choose one site to move over to the VPS to start, see how it goes, and if you ultimately hate it, drop it and move it back.
I’ve been with Dreamhost for 7 years now? And I love it. There isn’t anything I haven’t been able to do using their Control Panel and the number of times I’ve had to contact support or needed them, they responded timely and resolved my issue entirely. I’ve only had one major outage with them, and because of the size of my setup, it took a very long time to restore it, but they gave me a year of service for free for the inconvenience (which more than made up for it) – as it wasn’t a very important site, well not from a business perspective anyway.
Hopefully this answered a lot of your concerns/questions.