I'am interrested in this story because I might find my self in the same situation. Here is the strategy I applied so far.
1. Keep key aspects secret. One always has to release some info, at least to get some feedback on the pertinence of the general idea or when we sell it. But keep some trump cards hidden in your pocket. This is often related to the 'how' aspect of the idea where you could make a difference with a "stealer".
2. Proceed in a way allowing you to prove that you shared the idea with X or Y. This is one of the purpose of an NDA (non disclosure agreement). If you can proove that, then you have a leverage on the stealer's investors because you may publish it and it will call back into question their honnesty and fairness. If they ripped you, they could rip clients. This is why public opinion frowns uppon stealing ideas even if it's not illegal.
3. If option 2 is not possible, then another leverage you have is that you can publish the idea and put the investors at risk to face many competitors and eventually freeware versions of it in a very short time. This is even more effective if the product is not out yet.
Be aware that you represent a direct threat to investment when comming up with such kind claim and want to use these leverages. So prepare your negation before using those leverages. This means that directly after presenting the problem to the investors, present the different options you have at hand. First present the options that would require some sort of cooperation from their side, second the only options left to you if they refuse (the one I presented above) and third that you are open to any other options that they may want to suggest and that would respect everybody's interrests.
Be aware that there are things that can't be undone and also that it is better to have half of something than all of nothing. You can remind this rule while argumenting your cause, but keep in mind that it applies to you too. So be ready to accept an arrangement if possible. Keep a very open mind about the options you will consider. For instance you could ask them to invest in another idea you might have, but be very carefull to not blackmail them into that. Just say that this would be an acceptable outcome for you if they agree to it and that you would then drop the case because you don't bite the hand that feeds you. Of course this suppose you have a valid alternate idea to show them when you say that. But be carefull with this because VC can easily get you out of your founded business if they want to. What you have to sell to VC is your capacity to create value by your inspiration or vision. This is what they are looking for.
There is alot more to say, but PG is much better than me at it.