Is there anything special that needs to be done to plug a thumb hole that also has a vent hole?

Is there any particular reason to use a vent hole?  

I have 2 balls that I bought that have vent holes.....the span feels really close, but the thumb is slugged and way too big.  

Would I be better off:
 a)having the slug drilled out, plugging the thumb and re-drilling, or
 b) having a new slug put in and drilled out to fit my thumb?  I've never used a slug before.
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