Most people will say NDAs are dumb and reaching out to VCs on the condition they sign an NDA is a quick path to their "do not reply" list. For most app ideas, this is true - execution is what matters. However, if you have serious intellectual property, patents filed, something that is a big breakthrough - only then are NDAs warranted.
VCs are not in the business of stealing ideas. But as a founder, if your startup is successful you will undoubtedly face competition eventually. This is where the true protection of your business comes into play - by building a better product, with better execution.