So I looked at the code for the vaccum bulb and then looked at the tags for the shadow pieces and fuelweaver, I can confirm the vaccum bulb does not deal 2x damage to them as it does shadow creatures, it doesnt even effect maxwells shadow minions, which it should.

If you want the weapon to work against this(as it should) then heres the tags it should detect.

for Shadow Pieces the tag would be "shadowchesspiece"
for Fuelweaver/Ancient Skeleton the tag is "stalker" the minions being "stalkerminion"
for Maxwell's shadows the tag is "shadowminion"

I feel for these tags the vaccum bulb should get its 2x damage bonus.