I prunned my vallis which was about 1m long about a week ago. I cut the leaves straight across with a new razor blade (very sharp). Non of the ends have so far turned brown, but I am keeping an eye on it.
If you are worried about the end browning then you could cut them off near the crown of the plant although there is a risk of die-cack affecting the crown and so the whole plant. I don't know, it's just a suggestion!
WK