for differences, dd performs a low-level byte copy of the disk, whereas vxdump copy at the filesystem level. Either one will get the job done for you. If you will have both filesystems mounted at the same time, I would use find . -print |cpio -pdm /mntpnt.
Second, you are planning on dumping to an intermediate tape and then using vxrestore, right?
Given that this is exactly what vxdump/vxrestore was created for, I would lean toward using them. No idea on the relative speed of each approach, however. It might be interesting to do some comparisons.
dd is best suited for RAW copies -- that is if your target and source disk or LVOLS or VOL are sized exactly the same. dd is your only means if you have data that is actually stored raw .. ie. Oracle and Sybase instances that use raw devices.
vxdump/vxrestore is the safest IMHO if transferring/migrating data accross filesystems as it takes care of links (bad or good), permissions and ownerships... cpio I will use at directory level copies although latest versions of VxFS (3.4) already supports dumping directories instead of whole filesystems.
