Legal recourse after VC disclosure without NDA?

Obviously NO.
Sign the NDA first.
Any idea becomes a success, only when all things come together, funds, team,product roll, marketing, and promotion etc.

I have never seen an investor sign an NDA. Investors, like many business people, realize that an idea's "time has come" and that, in a world where innovation is coming from every corner, you are most likely not the only one thinking about the same thing. Further, investors will not want to box themselves into an NDA with you if it could cause problems with them investigating similar technology with someone else. Finally, even if there was an NDA in place and you had recourse, would you have the legal wherewithal to actually do anything about it, if they did commercialize your idea without you? Your best option is filing some sort of patent application before investor meetings. You do not need to have a finished app to file a patent application, you just need an enabled specification on how to build the app and practice the invention (this is assuming that you have a new and novel invention). Unlike an NDA, if your patent application eventually turns into a granted patent, it is a piece of property that has tangible value that can be bought, sold, licensed, etc.

Gustavo referenced what in the US is called a provisional patent. It is filed as a specification with drawings. It MUST not have anything resembling a claim or the USPTO will consider it a non- provisional application. Provisionsl patent applications require fewer and lower fees, and don't themselves become patents. They have a 12 month life and their primary purpose is to establish a filing date for a patent application. Normally you later (within 12 months) file a non-provisional that claims the provisional filing date. So they save up front money ( but add a bit to overall expense) while securing investor funding. If you are a smell company (<500 people) or an individual you may get 50 or 75 percent discount on filing fees as small or micro entity. However, don't skimp on the spec and figures. The later provisional can claim only what is in the provisional when that is filed. Your spec should be nearly as complete and thorough ( example- rich with many alternative embodiments ) to give a solid foundation for the non provisional. You can always sell either if the product or company doesn't pan out