Welcome to TFP!!
I think what Ann (frustratedpoolmom) is trying to get at is to check for other possible solutions before suspecting a faulty line. While it's possible to have a leak in the suction line, you'd probably notice air in the pump even without the vacuum attached - though using the vacuum would exacerbate the air.
How and where do you attach the vacuum? Do you have another suction line, like a main drain or skimmer? If you're vacuuming through the skimmer, all you may need to do is turn the hose around (many vac hoses have 1 end with a swivel cuff, that end will draw air and should be on the vacuum head)
Let us know a little more about this and we'll help you decide if a leak in the line is probable and, if so, how to test the line. :-D