I had just about the same situation earlier this year. I had forums that were outgrowing the reseller hosting I was using, and a VPS was recommended as a fix. After doing some research, I found that the VPS market is divided into two segments: managed and un-managed.
Un-managed VPS are bare bones boxes, with just an operating system on them, such as CentOS. You have to either buy a control panel or log in and install the standard LAMP stack via SSH; it is not a trivial thing if you have never done it. I actually think it is unethical to experiment with your client's data, so I knew I had to go managed.
A managed VPS is very similar to a reseller account. There are plenty of differences; the WHM control panel is more complex, but you can get up and running quickly. I was also concerned about having a secure backup of the sites with daily restore points. I found that many managed VPS providers offer backup with daily restore points. I found a good home at MDDHosting.com after a couple of false starts, and ran parallel for a month, moving over my sites first and then customer sites one at a time.
I decided on a "standard" configuration of CentOS, Apache, MySQL and PHP, with cPanel/WHM to have the most compatible platform possible. I wanted 1 GB of RAM to allow MySQL tweaking for more cache. Could my 30 sites run on less RAM? Probably, but it is a very nice platform now, and my customers appreciate it.
Even after all my research, I found out a few things that surprised me.
My prior reseller account did not have the MySQL databases as part of the account quota. I found it did make a difference on the defined disk space amounts I had for my customers. Also, the software previously mentioned takes up disk space, so allow a couple of GB for it. Softaculous, which no one was using, took up 2 GB of disk. I removed it since my customers are almost always SMB or NGOs that want me to handle everything for them.
I installed CSF / LFD, a software firewall that has a GUI that hooks into WHM as a plugin, and found that there were dozens of intrusion attempts each day; it took a while to configure things so the server was more secure. Having a managed VPS made that much easier, as I could bounce things off support as I ran into them. I could have had them install it from the beginning, but I wanted to learn. Having a managed VPS gives you "training wheels" while you learn.
One issue you will have is that your VPS will have to have the authoritative name servers on them. Your "ns1" and "ns2" will resolve to the same server, so you will get warnings from places like http://intodns.com
that you have a "single point of failure". This impacts email delivery if your server has a maintenance window ... email gets bounced back to the sender immediately. If one of your name servers is on a different server, the email gets held in queue for 4 to 48 hours, usually long enough for the main VPS to come back online.
My solution was to buy a $15 a year VPS from BuyVM and install cPanel DNSONLY on it, and use it as the remote authoritative name server. Problem solved; now I have two distinct servers handling the name server requests, and if my main VPS is ever down, email is queued instead of bounced. My SMB and NGO customers like that; they don't want their customers thinking they have pulled up stakes and moved.