I would have to disagree which is harder.  A urethane slug is much harder than a vinyl slug anyday.  If you drill a vinyl thumb slug, it is very soft and doesn't give a good feel, plus it is very hard to shape and bevel.  Also plug material is a 2 part mixture that the end result is the same hardness of the ball material, in most cases urethane which makes up are resin balls and just as hard as the plastic that makes up a Target Zone.  If you want to know what feels best to you, go to the pro shop and see if he has a thumb drilled with a vinyl slug and a urethane slug and see which feels better to you.  My experience would push me to the urethane slug which is hard and consistent.  Just my opinion and experience.

I'm too young to use 45s or L.P's.  I just know I can bend a vinyl slug in my hands and not a urethane slug.  End of story Grandpa.

Happy drilling

I know it has already been answered. but usually players what to use vinyl only for the reason that they want to slow there release down and they come out too fast with urethane slugs. Urethane is the better material IMHO. There is still many shops around using vinyl oval inserts for bowler that have oval thumbs but don't have a milling machine that can do it.
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Timothy @Juniors Pro-Shop
Staff Writer 7-10 Split Magazine,EGO Communications
Montreal, Quebec.