You must also have a partition that has at least enough space to hold the /var/www/vhost content...
Plesk migration manager stores dumped file under /home/migration. To alleviate the risk of filling the / or /home partition do this:
CODE
rmdir /home/migration
ln -s {otherpartitiondir} /home/migration
ln -s {otherpartitiondir} /home/migration
where {otherpartitiondir} is a full path on the partition that has enough space.
I hope that will help you as Plesk is not really telling you where the stuff is stored (or I did not find it)