Well Liz, I have done this several times. My vent pipe is sheet metal, and can be cut with tin snips. Word of caution. Sheet metal cut with snips is as sharp as razor and can cut you very quickly. Suggest using work gloves.
By the way, I used a circular to rectangular transition above the ceiling and a narrow rectangular duct in the cabinet so as not to use up so much room. I think the duct is 2-1/2 by 10. The fitting on the microwave was cut for rectangular duct.