I have had success on this a couple of ways.  

You can put them in a ziploc first, but don't seal it up.  That gives you an extra layer of padding against the sharp points.  

Lately, I have had success by skipping the ziploc layer and just not vacuuming quite as much air out before sealing.  I let the vacuum part of the process run just until the stuff compacts down fairly good but before it starts putting a lot of pressure on the bag.  Then I switch it over to the seal function.  You get 80-90% as much compression as usual but just enough "give" in the bag so that the pointy edges aren't straining against the plastic.