I think your problem is mainly one of LVM filtering ("filter =" in /etc/lvm/lvm.conf).
LVM is probably seeing the /dev/sda device before it sees the /dev/vpatha device so that is what it is using.
(BTW, the devices you see in the files in /etc/lvm/archive and /etc/lvm/backup are just what they say. They're hints. LVM works off the metadata written to each block device it controls. Since /dev/sda, /dev/sdb, /dev/sdc, /dev/sdd and /dev/vpatha are really all the same device, the metadata is the same.)
You should be filtering out at least the underlying "/dev/sd" devices. There are good comments in /etc/lvm/lvm.conf.
My guess is you have the default filter.
# By default we accept every block device:
filter = [ "a/.*/" ]
If that's the case, you could probably get away with
filter =[ "r|/dev/sda|", "r|/dev/sdb|", "r|/dev/sdc|", "r|/dev/sdd|" ]
When you do a 'pvscan -vvv' you need to see somewhere in the output this
/dev/sda: Skipping (regex)
/dev/sdb: Skipping (regex)
/dev/sdc: Skipping (regex)
/dev/sdd: Skipping (regex)
Hope this helps.