Vacu-Grips aren't any softer than a regular insert...it's just a method for allowing an undersized grip the "give room" necessary for a bowler to fit their fingers while still being attached to the ball.
I'll have to check with a friend of mine, but I recall him using grips that felt and looked like silicone. They were super soft and tacky...but they didn't last very long.
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-Chris: DJ's Pro Shop : Auburn, MA