There is no "Audio Event Velocity" in Vegas Pro. Best approach for audio you could get is using the "élastique Timestretch" as Track FX, then automate the FX to make it adjustable via keyframes. But this is cumbersome at least, lots of trial and error if gonna be used to synchronize to a video speed ramp.
Unfortunately that will not work, as the video is ramping from 0% to 100%. The audio would only stretch at a fixed rate. Temporarily, I have cut the audio into little pieces and created the velocity ramp manually, but a trained ear will notice it. The average person hopefully won't notice...
VP17 will not let me keyframe the properties of the Audio Event FX. I tried both the "ExpressFX" and "Time Stretch" plugins. Looks like I am stuck with manually cutting it into pieces.
"VP17 will not let me keyframe the properties of the Audio Event FX."
That's why I said you need to use it as Track FX. And neither "Express FX" nor "Time Stretch" are keyframeable. You need to use "élastique Timestretch" as mentioned above. And you need to use the Track's automation mode.