Are you talking about RBG exposure slider in Input tab or Brightness slider in Color tab? RGB exposure is "hardware" exposure (longer/shorter time the sensor will read one line of data before moving to the next line) and Brightness is post scanning "software" adjustment.
Now, if you don't lock RGB exposure in Input tab, Vuescan will adjust the exposure based on per frame basis so it doesn't blow the highlights (blacks on negative film). Since individual frames might not include just base fog (pure black) Vuescan will bump up the hardware exposure (probably to be sure to get through the densest parts of the negative - highlights). Now, since Coolscan is a good scanner it is highly unlikely that it will have problems with negative film density even at default hardware exposure (1.0).
I'd just tick the "Lock exposure" box and set "RBG exposure" at 1.0 (or a bit more if you notice that your highlight are clipped) in the Input tab. One benefit is that you will get an approximation of an index print (Vuescan won't try to change exposure at scan time to lighten the underexposed or darken the overexposed frames) and the other is that the scan times will be reduced (always welcome as film will stay cooler and flatter that way).
Many will suggest to lock exposure on a blank frame, but I find it unnecessary with good scanners (16bit and good DMax).