can't start kdump service on virtual machine

# service kdump start
No kdump initial ramdisk found.                            [WARNING]
Rebuilding /boot/initrd-2.6.32-504.23.4.el6.x86_64kdump.img
No module vmmemctl found for kernel 2.6.32-504.23.4.el6.x86_64, aborting.
Failed to run mkdumprd
# lsmod | grep vmmemctl
vmmemctl        13966 0

Fixing VMMEMCTL module issue:

You can disable this module by editing  /etc/vmware-tools/locations and changing answer VMMEMCTL_CONFED from yes to no.

More general approach:

More general way to handle missing modules is to ignore the ones which can not be found:
Edit /etc/sysconfig/kdump and set MKDUMPRD_ARGS="--allow-missing"
# service kdump start
WARNING: No module vmmemctl found for kernel 2.6.32-504.23.4.el6.x86_64, continuing anyway

Configuring kdump in RHEL

kdump may be very useful for you if you're experiencing kernel crashes and want to analyze what is really happening there (or you may pass this part to support).
First install kdump package:
# yum -y install kexec-tools
Then modify /etc/grub.conf and add "crashkernel=128M@16M" after "root=LABEL=/" for current kernel.

It's necessary to reboot the box to make the new settings active:
# /sbin/init 6

After the reboot make sure that kdump service is enabled and running:
# chkconfig kdump on
# service kdump start

You should also test if your setup is fine. Open console and enter the following command to trigger the dump:
echo "c" > /proc/sysrq-trigger