A cracked oil filler tube does cause a vacuum leak. However, may not be the only leak you have. Only way to tell is replace the tube and see.
Replacing the tube is a pain. It's best to buy the hose clamp tool. I got mine from Sears for $40. I believe it was a Craftsman. There may be others out there that are cheaper.
Lots of folks replace the OEM clamps for wormgear type hose clamps. But after you buy the tool, there's really no additional benefit of replacing the OEM clamps. Just personal preference.
Oil filler tube
Hose clamp tool
Replacement instructions/pics