Unable to boot GRUB legacy

[omerule]

LS,
I have a CentOS 6 installation disk with 3 partitions:
1) /boot
2) /
3) /home
After given the command memtest-install and do a reboot I got a prompt grub-rescue>
I tried to fix it with a live fedora 25 after reboot I got grub>
Then I tried to boot from the grub> commandline I see that the kernel is loaded but the system reboots after the kernel is loaded.
Then I knew that I broke the installation because I installed grub2.

How can I restore the original grub legacy installation?

With a virtualmachine I installed the same setup as production from above, only now with two disks:
vda and vdb
first I installed centos on vda:
vda1) /boot
vda2) /
vda3) /home
Then I rebooted and reinstalled centos again only keeping de original vda2)/ and vda3)/home, but format vda1)/boot
and on vdb I created a new partition schema:
vdb1)/
vdb2)/home
Hoping to keep the data of vda / and /home intact but reinstall grub etc..
I rebooted with the new install a combination of vda1)/boot and vdb1)/ vdb2)/home
Then I changed /boot/grub/grub.conf boot=GUID={guid of vdb1} > {guid of vda2} saved the file and reboot.
But then I got a error not finding.... and /boot was empty

Is there a way to adjust grub legacy to choose not vdb1 but vda2?

Yours kindly