VxVM vxdisksetup ERROR V-5-2-5716 Disk c1t1d0 is in use by ZFS.

Haha, this is a stupid one since the error message contains the solutions already.
If you ever want/need to reuse a disk that was used bei ZFS prior as a disk for VxVM then you might encounter the following error:

# vxdisksetup -i c1t1d0
VxVM vxdisksetup ERROR V-5-2-5716 Disk c1t1d0 is in use by ZFS. Slice(s) 0 are in use as ZFS zpool (or former) devices.
If you still want to initialize this device for VxVM use, please destroy the zpool by running 'zpool' command if it is still active, and then remove the ZFS signature from each of these slice(s) as follows:
    dd if=/dev/zero of=/dev/vx/rdmp/c1t1d0s[n] oseek=31 bs=512 count=1
    [n] is the slice number.

The error message holds the solution. Just run dd on the ZFS disk:

# dd if=/dev/zero of=/dev/vx/rdmp/c1t1d0s0 oseek=31 bs=512 count=1
1+0 records in
1+0 records out

Then try to run vxdisksetup again, it should work without any error now:

# vxdisksetup -i c1t1d0