It would probably be better to recreate the LUNs, and make good backups.
I wouldn't trust a LUN that has been "rebuilt".
The answer is 20 Mb and is covered in the VMworld 2007 session TA 48 Advanced VMFS Configuration & Troubleshooting, as is the process for backing them up. http://www28.cplan.com/cbv_export/PS_TA48_288131_166-1_FIN_v2.pdf
This link requires username/password? Could you please post the procedures to backup and recover?
does dd=/dev/sdx of=lun-header-20MB bs=1024 count=2048 work?
create a vm-support and stash it away for later. This includes partition table info, etc. that will come in handy if you need to restore
backup .sf from the root of the lun (cp /vmfs/volumes/<name>/..* <dest> as there are files starting with .)
locate Lun ID: vmkfstools -P /vmfs/volumes/<name> |grep vmhba. Let's assume that comes back vmhba0:0:1:1
find the SCSI device name: esxcfg-vmhbadevs -q |grep vmhba0:0:1 Note this has the partition number removed, e.g. last :1 from above. Let's assume this returns /dev/sdd
dump the first 20Mb: dd if=/dev/sdd of=lunhead.bin bs=1024 count=20480