I'm not familiar with Virtualbox (and suspect that this is the case with others, as there have been a lot of views, but no responses), but I'll take a stab at this.
Your 400GB RAID1 /dev/md0 (as you said, composed of /dev/sdb1 and /dev/sdb1) is being mounted on /media/VCMIRROR.
If you want it mounted elsewhere, you need to first unmount it: 'umount /media/VCMIRROR' or 'umount /dev/md0'.
(You can umount it by device name or mountpoint by virtue of the fact that there is an entry in '/etc/fstab'.)
Once unmounted you should be able to mount it where it will be useful to Virtualbox. There I can't help you,
but it would be in the form 'mount /dev/md0 /<mountpoint>'
Once happy with where you've mounted it, don't forget to update '/etc/fstab' so that it is mounted at boot time.
By the way, to check that your Software RAID is healthy, there is a pretty obscure method: 'cat /proc/mdstat'
I've got two Software RAID arrays on my system
[root@athlon5k2 ~]# cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md1 : active raid1 sda5 sdb5
1440026304 blocks [2/2] [UU]
md0 : active raid1 sda2 sdb2
25005056 blocks [2/2] [UU]
unused devices: <none>
If you have questions or problems with Virtualbox, you can post a new question with Virtualbox in the title. That might get a better response.