How do I login into VE without knowing its root password?
There are many ways for a hardware node owner to login into a VE or execute a command in VE context.
# vzctl exec 101 /bin/ls
will execute '/bin/ls' command inside VE #101.
# vzctl enter 101
will run a shell in VE #101.
If you need to log into VE #101 as root by SSH you can add a user with UID=0 to /vz/root/101/etc/passwd and set a password for this user using
# vzctl exec 101 passwd username