I tested this and it seems to be working OK.
I presume you're talking about inserting velocity envelopes manually using Media | Velocity Envelope, rather than the automated ones from the event tab during exit and deployment.
The event has to be split to allow the velocity envelope to be applied. If you add a three second 25% velocity event, you'll wind up with three events total. Normal speed event, then a 25% speed event of 12 seconds, then normal again. There are no transitions between these three events but the velocity envelopes are ramped so the slow motion is applied gradually.
The audio for freefall between exit and deployment is muted so the audio setting applied for the velocity event will be ignored.
Mick Hardy attached the following image(s):