smv,
According to the VxVM 5.1 Administrator's guide p136 (pdf link here: http://sfdoccentral.symantec.com/sf/5.1/solaris/pdf/vxvm_admin.pdf )
"On Sun x64 systems, VxVM automatically creates a GRUB menu entry for the alternate boot disk when the boot disk is mirrored. During the booting process, select the alternate GRUB menu entry from the system console.
An alternate method is to change the 'default' GRUB menu setting in the /boot/grub/menu.lst file to select this entry automatically during the booting process."
vxbootsetup is used to install the bootblk/setup the disk as a bootable system disk - however according to the man page this is called by both vxmirror and vxrootmir automatically:
http://sfdoccentral.symantec.com/sf/5.1/solaris/manpages/volume_manager/man/html/man1m/vxbootsetup.1m.html
Unfortunately I don't have an encapsulated 5.1 x64 machine to test/verify this, however the documentation does suggest that if vxmirror or vxrootmir have been used to mirror the rootdisk (ie: not vxassist mirror), that implies the bootenv and grub menu should have been updated as part of this process?
Just to be clear, which of these is the current scenario?
a) You are currently putting together a procedure to mirror using CLI instead of VxVM, so you are just trying to determine the correct equivalent commands
b) you have actually attempted mirroring via CLI (vxrootmir/vxmirror), but the bootenv.rc and grub menu list were not updated ie: you obtained different results to what was detailed in the documentation
Just trying to clarify whether you've actually had a problem, or you're asking for clarification/more details before attempting the procedure on your machine.
EDIT: the following from the VxVM 5.1 Troubleshooting Guide may also help you:
Manual GRUB procedure to set up an alternate boot disk
http://sfdoccentral.symantec.com/sf/5.1/solaris/html/vxvm_tshoot/ch03s03s02s03.htm