Search for Hidden City Ticketing, hundreds, if not thousands of threads about it.

No matter how many times it's asked, the answer is pretty much always going to be the same. Skip a segment, and any flight segments you have on the same ticket will most likely be canceled.

If your ticket is from

LA to Amsterdam to Vienna

and returning

Vienna to Amsterdam to LA

once you miss the Amsterdam to Vienna segment, the remaining segments may be cancelled. Most likely will be cancelled.