At first you need to unmount the second device again, before you proceed with the following steps:

You will have to add the device /dev/vdb into your logical volume group VolGroup, you can do this using vgextend.

vgextend VolGroup /dev/vdb
After this you can first grow the logical volume lv_root to the size of the group, using lvextend.

lvextend /dev/mapper/VolGroup-lv_root -L+49G
Then you need to grow the file system on the logical volume with fsadm.

fsadm resize /dev/mapper/VolGroup-lv_root