Admin CPU issues

Hate to say this, but 1gb RAM is just barely enough. You were better off with the shared server.

Seriously, by the time the server has loaded several instance of Apache, plus the SQL server, along with a mail server, DNS resolver, SSH, possible a Cpanel, there isn't going to be much left over for data manipulation and storage.

I'd suggest 2gb as an absolute minimum. 4gb if you want good performance.

No, you are not 'out growing' zencart.
You (like waay too many others) have made the mistake of going from a cheap shared host (cheap compared to dedicated hosting) and "updated" to a low end VPS without realising that the update is actually a downgrade in resources.

A shared hosting could have up to 16 or more Cores and over 64, even 128gb ram available for "use on demand ' (as opposed to constant use).

I place most of the blame for this on the shared hosting providers, that often survive on the overselling the shared servers, identifying the sites that become popular, then upselling the VPS offerings to the "resource hogs", who invariably think that they will get a better performance for the higher price, and then find themselves where you are now.
By this stage, most people are well and truly "hooked", and the formerly very cheap host selling overpriced VPS, with full knowledge that most people either won't shop around for better value, or they just find it "too hard" to bother to migrate.

Alas, a busy site is a busy site, and they do use lots of resources, there is no way around that. Sure, you can tweak and optimise various things to get the most of what is available, but rarely enough to make a significant difference, and the site will (hopefully) only get busier and busier. Might as well save the effort and just add more RAM.

No need for to bring in an outside consultant. You just did. :)

Cheers
RodG

I agree with Rod's assessment about your server.

You're not outgrowing Zen Cart. You've outgrown your server.

Also, see: Webserver Tuning Tips

Stats from Munin that I activated yesterday afternoon. So, what I'm getting is that while we resolved the issues of going over allotted process counts pretty consistently (often up to 30 b/c of IMAP, allotted 25 before things panic) by switching to a VPS, nothing else got resolved, and I'm now getting longer load times on my admin.

Yup, you're maxing out your memory, and It looks like you don't have a swap file/disk enabled.

You will probably get a performance boost by creating a swap file, but you'll get better performance with another half gb of ram. You are just touching the boundary.. If pushed much further (without a swap file) you'll find that some services will start to kill themselves to make more ram available for the essential services. This could possibly be happen already (check you kernel log files)

Cheers
RodG

Q. Are you running a 64 or 32bit Kernel?

If 64bit I suggest you replace it with the a 32bit one - The 64bit kernels use a lot more memory, and unless you have over 4gB RAM it is doing more harm than good.

Just a thought/suggestion.

Cheers
RodG

ps: Also just noticed a bit of an oddity in the graphs and the associated data.

The 'buffers' values are all at zero, but the graphs indicate this is not the case.

It is very unusual to have a buffer of zero, so I'm assuming the graph is correct (where is it getting its data for this?).

If your buffer really is at zero that is going to slow things down a lot too. I'm going to suggest a reboot to see if the values/settings get reset.

Cheers
RodG