Do not cap off the vapour return/vent line. It 'may' cause vapour lock in warmer weather and it'll be hard to diagnose the fault due to it being a minor mod that doesn't seem to have any real ramifications. Keep the charcoal cannister connected as this the safest way to vent vapour into the intake. Cap off the fuel return line though*. This will prevent any risk of moisture or junk entering the line and possibly making it's way back to the tank.

*depends on what variant of Weber you end up installing. The majority of Webers do not have a return barb fitted to them, but if you have salvaged a used unit from somewhere it may have a return on it...