I would play around with how CC works in vegas, read the help file and what not. CC can be toggled off and on, but the player that is embedded into the powerpoint, how the video is embedded in powerpoint will determine how this works.
CC can be embedded into come codecs (generally broadcast codecs)then all you need is the right player that understands that codec.
YouTube embeddeding will allow toggleing the CC, but YouTube does this different. YouTube shows you the mp4/VP8 video on screen, then has a player that pulls the CC when needed from a separate source and syncs it on top of the video playback. YouTube's implementation of CC creation is very good, and you download the CC you create as a file when done for use in other applications like DVDA. YouTube videos can be set to unlisted or private, so as not to be publicly available.
So all this really depends on how you plan on showing the presentation. Embedding YouTube into PowerPoint is something I would try, and see if that allows you to do what you want.