Just taken a look at an implementation of VAST ads at http://www.html5vast.com/
The answer is yes, it would be possible to implement VAST in to videos - it looks like it targets the ID of the <video> tag, and then plays the ad before the video itself is played. I can't help with the integration of VAST itself, but I can make it easier for you by adding an ID to each video that matches up to the ID of the post within which the item is played (example: this post is ID number 2717066, so I'd make the video's ID nevid_2717066...I'll add in something to handle multiple videos in posts too.... nevid_2717066_1, nevide_2717066_2 and so on)
Then in your VAST implementation, you could target those IDs.