After creating a RAID1, the installation works pretty well and no errors are reported.
However, on startup the RAID seems not to be found.

This seems to be an old issue: http://blackbird.si/installing-vyatta-6-6-r1-with-software-raid-1/
Using install system one could basically re-run grub and it would work.
However in 1.2.x, install system is not supported and it becomes more complicated to fix it by hand.