In general, if you are meeting a VC who's funded a competitor, we generally advise a few things. First, if you are fundraising, we'd recommend scheduling the meeting as the last meeting in your pipeline of investors -- basically so that you meet with them if the other investors don't bite and you don't have alternatives. In this case, if you don't need the money now, you can hold off and schedule this meeting near your Demo Day date.
If / when you do meet with them, you should set an agenda for the meeting that's similar to a Customer Discovery Meeting. That is, you should say upfront that you can share with them information about your vision and approach, but before you do you'd love to ask them a few questions to get their insights. Have them consent to that, and then ask them questions you are curious to get their insights on re: the competitor.
Towards the end of these questions, you should also ask them how you should think about talking to them given they have funded a competitor. And decide based on their answer how you want to proceed.
In general, when you get to your part to share, you can choose what you want to share, but I'd advise keeping it very high level. E.g. just explain you are still figuring things out, and maybe just talk at a high level about the team, pain point, and vision -- but not more details. And if they ask questions you don't feel comfortable disclosing, explain that you aren't ready to discuss that now, but they are welcome to come to Demo Day. Or say that you are still trying to figure out the answer to that question yourself and curious to get their perspective.
You can also set expectations before the meeting that you are not actively fundraising -- your Demo Day is when you will start fundraising -- but happy to meet now to get to know them if they are open to have an informal meeting to share perspectives.
In some cases, VCs will fund competitors and it's fine. This is typically when there's different partners involved w/ different companies, and there's a Chinese Wall between them.
But in others, a minority of VCs will be bad apples and share intelligence to your competitors, so you should not share confidential info -- e.g. customer names, etc. -- you don't feel comfortable with until you understand how they are handling the competitive nature of you vs their existing portfolio company.