Hey,
You can use a wave editor like Cool Edit to do this:
1. Load the sound you want to 'vinylize'.
2. Go to Edit>Convert sample type.
3. Choose mono, Left 50% Right 50%, hit ok.
4. Go to Effects>Filters and remove some of the low end, around 150hz, and remove some off the high end, around 15khz. Hit ok.
5. Go to Effects>Amplitude and choose Dynamics Processing.
6. Choose one of the presets and preview it, the goal is to find a preset which kills some of the dynamics of the audio you're working on to imitate the limited dynamic range of an over-played record.
7. Now that you have the file, save it and drop it into the multitrack.
8. Create a new file in the edit view and record a bunch of the silec parts at the beginnings ends of records (the turntable hum, crackles and pops are what we're interested in). Save them and drop them into the multitrack.
9. Go into the multitrack, loop duplicate your main audio file and delicately mix the different samples of the vinly files.
10. When you're satisfied, go to Edit>Mixdown and mix all the files together.
It's not perfect, but it's a darn site more convincing than just a simple vinyl plugin because you're replicating the acoustic qualities of vinyl, not just adding vinyl sounds.
Nick