One aspect of "telling the VP professionally" which hasn't yet been touched on is that your professional responsibility right now is to your organisation. For sure you need to present your feedback in a way which is fact-based and clearly not driven by personal animosity. But you do need to make sure your employer doesn't make a major mistake.
So when it comes to professionalism, it's important that you tell your employer everything. Unlike other answers, I suggest this should also include speculation, where that speculation is well-founded, and so long as you make it clear this is speculation. You can't say for sure that this person failed their PIP, but you can for sure say that you know they were on a PIP and were subsequently fired, and say that your conclusion from this is that they failed the PIP.
Part of "professionalism" is trying to stop your organisation from making mistakes which affect their products or reputation. Technically, that's might be giving useability feedback on an app so that you don't ship something clunky which hurts sales. But in this context, the mistake would be hiring someone who you know is a liability. It's totally possible that this person has put a lot of work into doing better, and they're now competent. But this warning means an interviewer (which should not be you!) needs to see the positive change in their abilities. The interviewer should also be looking for the person to freely tell them about making this positive change, because that's the kind of learning story you talk about in interviews. If they don't mention the fact they were on a PIP, that should disqualify them for being hired because they haven't shown that they've changed.
All of this means that the VP needs to know everything you know.
You don't need to worry about slander or libel, because this information is not being published or passed around more widely. Company-internal information is inherently not public and hence laws like that don't apply. If you're still concerned about this though, consider giving verbal feedback to the VP. Whether they write it down or not becomes their responsibility after that.