Vserver does do quite nicely -- We run 5 different vserver OSes on one machine including Debian, Mandrake, Redhat, Fedora and Gentoo (for a development project). Security-wise, it is fairly decent and the servers are fairly isolated.
UML in production use requires a lot more hardware to perform the same job. You have more control over Security and Resources. With SKAS mode, you can eek out a bit more performance than plain UML.
Then there's VMWare, which we have found to be great for development using multiple OSs. I wouldn't attempt to run it for any production mode servers.
We run 2.4.27, tux and openwall using XFS and a few kernel tweaks. I think your config will work just fine for Vserver. I've had some strange issues with Hardware and Software Raid1 & SATA with 2.4.26. I've not tested that particular combination with 2.4.27 and won't be doing SATA Raid again for quite some time.
