"Cross-buy works when you insert your PS3 game disc into your PS3, you follow the on-screen instructions to receive your PS VITA version, as a download from the PS Store. It’s tied to your PSN ID so you won’t be able to give away the PS VITA version to a friend."
It shouldn't come as a surpise to anybody, but this is the first I've seen of an official explanation of Cross-Buy. It's nothing different from how I expected it, and I assume many others are in the same boat, but for anybody who expected something different, there it is.
You are right though, there seems to be a very easy way to get around the disc based download - essentially it's just to stop people from selling codes, this way they'd need to sell their whole game which people ofcourse wouldn't want to do. Simple solution is launch the disc and redeem the Vita version on the PSN account who will be playing it on the Vita and then enjoy your PS3 copy as normal. Assuming an online pass is required for this process (if there is any), you'll have to use the online pass on the PSN account that wants the Vita and then either keep their account activated on your PS3 to game share the online pass or simply repurchase the online pass from the PS Store.