How do I remove a wiffle ball from a pool vacuum pipe?
You are going to need to "pig" the line. You will need to insert something very slightly smaller than the pipe diameter (so it doesn't get stuck!) into the pump side of the pipe (i.e. where the whiffle ball wants to go "to") a tennis ball or racquet ball might work, so might just some crumpled up news paper. Seal your pig into the pipe and rig up a fitting for the air compressor hose. Turn on the air compressor and your pig will travel to the blockage and then proceed to push it backwards out of the pipe.

You will probably spend $25-$50 USD on the fittings and such (depending on what you already own) but it is far cheaper than digging up the pipe and cutting it out.