This page lists various hints needed to resolve package update conflicts.
vzctl 4.0 and ploop 1.5
For RHEL5-based kernel (i.e. ovzkernel-2.6.18-028stabXXX) and earlier kernels
Since ploop is only supported in RHEL6 kernel, we have removed ploop dependency from vzctl-4.0 (ploop library is loaded dynamically when needed and if available). Since you have earlier vzctl version installed, you also have ploop installed. Now you can remove it, at the same time upgrading to vzctl-4.0. That "at the same time" part is done via yum shell:
# yum shell > remove ploop ploop-lib > update vzctl > run
That should fix it. In the meantime, think about upgrading your systems to RHEL6-based kernel which is better in terms of performance, features, and speed of development.
For RHEL6-based users (i.e. vzkernel-2.6.32-042stabXXX)
New ploop library (1.5) requires very recent RHEL6-based kernel kernel (version 2.6.32-042stab061.1 or later) and is not supposed to work with earlier kernels. To protect ploop from earlier kernels, its packaging says "Conflicts: vzkernel < 2.6.32-042stab061.1" which usually prevents ploop 1.5 installation on systems having those kernels.
To fix this conflict, make sure you run the latest kernel, and then remove the old ones:
# yum remove "vzkernel < 2.6.32-042stab061.1"
Then you can run update as usual:
# yum update