Crazily enough, the best reference I can find for a plastic safe keyboard lubricant is .. on a Talky Tina doll fansite.
TL;DR version -- make sure you pick a lubricant that specifically says it is safe for plastics, and does not contain petroleum distillates that will damage plastic. These may be easier to find at hobbyist shops than general purpose hardware stores.
I will also note that I have used white lithium grease pencils without incident to lubricate plastics in the past, which works well because it has a non-runny "crayon" like consistency that makes it easy to apply. But maybe I just got lucky, per below.
We can mail you a small tube of plastic-compatible grease for $3.00 plus postage. Vaseline is just about the worst grease you could choose, because any petroleum-based grease will dissolve at least some formulations of styrene. We frequently find white lithium grease inside when we open up talkers, but we're very reluctant to recommend it, since some lithium grease formulations contain petroleum distillates that may dissolve plastic. Some lithium greases have a synthetic grease base, so presumably those would be okay--the only problem is that (unlike food products) grease formulators aren't required to list the ingredients on the label.
The only grease we can recommend without hesitation is Super Lube, since that's what we've been using for the last ten years or so. (We haven't seen it in tubes in the stores lately, but Lowe's carries it in a spray can--try the Multi-Purpose, not the Dri-Film.) The important thing is that you use an all-synthetic grease, without any petroleum-based ingredients.
Or, at your local outdoors store you should be able to find small tubes of Abu Garcia's Silicote Reel Lube with PTFE (Teflon), for lubricating plastic fishing reels. This is what we'll send if you order from us. It looks like it's the same stuff as Super Lube, and the manufacturer says it's plastic-safe. When our current tube of Super Lube runs out, that's what we'll be using. It's very likely that Abu Garcia's reel lube IS Super Lube that they've put in their own tubes.
At Wal-Mart we found a fishing reel oil and grease combination packaged by Jadico/Laker. A phone call to the manufacturer indicates that the grease is the same as Super Lube.
At a bike shop we found Finish Line's Teflon-Fortified Grease. It's a 100% synthetic premium grease, so presumably it'd be safe too.
Your best bet for finding a plastic-safe grease, though, is probably a hobby shop--radio-control and model train enthusiasts can't live without the stuff. At the local hobby store we found Bachmann's E-Z Lube and Woodland Scenic's Hob-E-Lube, both labeled "plastic-safe." (We've heard the Hob-E-Lube is pretty runny, though.) Labelle Industries also makes a plastic-safe grease for the hobbyist.