The vacuum advance should always be used on a street driven car. You are losing performance and throwing your fuel dollars away by unhooking the vacuum advance. The advance is not an emission only thing. In high vacuum situations some street cars need up to 50 degrees of advance, how will a mech. only dist. fill that need.
You have full mech. advance built into the dist. It doesn't go away because you have vacuum advance too.
Real race cars don't need vacuum advance. Street cars do.
That pro bullit dist. is totaly wrong for your application. Look up some info online to see what a vacuum advance actually does before you throw away your money.