Going into PSN to get Limbo for my Vita so we could talk about it on our podcast, GO FOR RAINBOW! I discovered a button that said FREE where the BUY button should have been. At first I thought it was a glorious error, but as it turns out, not only is this a cross-buy title (a feature that was not well promoted) but the cross-buying works retroactively!
Looking at the comments below the post here a few days ago dating the games release on PSN, there seems to have been some confusion: in the comments people are debating whether this version is worth another $15 when they already have it on the PS3.
Worry not. If you bought Limbo back in the day on PSN, the sweet-ass Vita version is waiting for you with open arms and puckered lips.