Following my disappointing VPS experience, as a customer, here is what I learned in the few days following by doing some quick research and talking to different vendors. See this as a rough draft (with most information subject to inaccuracies). I feel like a should contribute back here since I've used the information many have posted in the past.
From a customers perspective, most VPS offers fall into 3 categories:
1) Random VPS provisioning.
These vendors decided to pretty much handle VPS accounts like shared hosting accounts. Most will over-sell and not care about the "quality" of the VPS. So you end up with a good or bad VPS and at any time your VPS can turn good or bad depending on the other accounts sharing the server. Many vendors will provision these VPS on machine that are old dedicated servers or even old workstations.
Since many get a VPS to install or develop custom software, one would think the risk of ending up having issues on a VPS are probably superior then a typical shared hosting account. Also it appears these providers use software like Virtuzzo which does poorly in term of truly protecting a given VPS from a "rogue" one. Like some pointed out, in the other thread (see previous link), you end-up with the aoftware stack OS(Linux)-Virtuozzo-OS(Linux)-OS(Linux) so for similar hardware, performance will be slightly slower on a VPS. However I will suggest that anyone with a VPS returning a time back > 1s is probably experience the consequence of this approach to VPS provisioning. Someone paying for a higher priced VPS with a time > 1s (when their own VPS is nearly idle) should probably ask to be changed to a different VPS or change providers. Beware most VPS providers fall in this category.
2) Engineered VPS provisioning.
These providers actually put the appropriate amount of VPS on the appropriate enterprise grade server. As time passes by, they also monitor these VPSs to make sure the customers get roughly the expected performance. They probably end up having to police the "resource abusers". Sort of what a shared hosting provider have to do regularly with resource hogs.
I do not think they are many provider that fall within this category in the "real world". Maybe the Wiredtree "managed hybrid" offer would be in this category.
3) Hardware VPS (the "right way" to do it).
These companies use a thin software layer (can even sometimes be embedded in chip, like the bios of your PC) on virtualization ready hardware (Intel VT and AMD-V hardware), to offer a VPS which will the an exact slice of a true enterprise grade server. It appears the lost of performance due to this type of virtualization is near 0 (Google "Citrix XENServer" for more information). Very few providers offer this type of virtualization because it really cost nearly more then the equivalent entry level dedicated server. Some of the advantage this type of VPS offers, versus an entry level dedicated server, is getting access to enterprise hardware (like Raid-10 sas disk, dual power supplies, hot-swap fans ..etc..) and the possibility to scale more easily (cheaper "virtual" cluster ...etc..). Cartika appears to offer this type of VPS. Although the VPS links (the "none clustered VPS" links) appear to have been removed from their web site (... might be my sleepy eyes).
In summary, besides solution 2) and 3) it appears the vast majority of the VPS market is appropriate for someone that can not afford a dedicated server (or wants to save money) and has a need to install custom software. Unless you need custom software, a shared hosting account has about the same risks as a VPS but for a cheaper price and your site will most likely end-up being a lot faster.
Hope this helps some people understand VPS a little better and make better business decision for their sites.
The only one I know for sure that does it is Cartika.
Although the "VPS plans compared" page (not the "clustered VPS") appear to have been removed since yesterday (might be their webmaster's mistake...).
Rackspace might also have an offer.
Your best bet might be to google it "citrix vps hosting", "xen vps hosting"
and "xenserver vps hosting" all seem to give good results.