When setting up a mirror on an empty volume, particularly a large one, there is no reason to go through all of the time it takes to make sure that the plexes are synchronized. If you're just going to build a new database or filesystem or whatever on top of it, this is just wasted time, and the larger the volume, the more time it takes.
VxVM has an option for initializing a mirror to think that it is
already synchronized (or doesn't need to be synchronized). Here is
a bit of code that will create your volume to create a simple mirrored
volume without going through the synchronization.
# vxassist make newvol 10m layout=concat-mirror init=active disk1 disk2
The init=active keyword is the key.