It compacts it more, just like buying a different harder tip.
Now, do you want that compression done by an amateur (you),
or would you rather have it done by a professional (tip company)?
Forget the vise, it will flatten the tip and make it into a pancake. If you want a harder Elkmaster tip, soak it with water then microwave on high for a minute. Repeat if it isn't solid enough.
Or... just bury it out in your back yard and dig it up in the spring. j/k!