Question
I have 2-25 foot Arbor Vitas I want to cut 5 feet off the tops,will this kill the trees>

Answer
It will not kill them but will not do them too much good. IF you want a hedge looking "tree" that is what you will have if you do this. Sometimes if you cut too much off the top you have dieback. You will more than likely be ok with cutting 5 feet off but I would expect getting some foliage damage from the cut.