Sorry, the envelope is active for the whole track. If you just need the lower the audio of a clip, that can be done without an envelope, but if you need to vary the volume during the clip, you will need to use an envelope.
In Vegas, if you create the audio envelope, which is track level only, over the event and then move the event, the envelope moves with it.
Unfortunately, there is no way to have envelopes (volume curves) at both the event and track level as you have seen.
Magix' other programs, Video Pro X (VPX) and Movie Studio (MMS) do have separate volume curves at object and track level. A volume or pan envelope on the track does not move with an object. The curve or envelope on the object/event, does, of course, stay with the object if it is moved anywhere else.
Another difference is that Vegas allows VST effects on events, whereas VPX and MMS do not. There is a limited way to do it use sends on the event to the FX strip in the Mixer.
It seems that we can't get everything in either program, unfortunately.
I have a perpetual license to Cakewalk Sonar-Platinum which does have volume envelopes at both track and clip levels. I understand the new owner, BandLab, has a free beta Sonar-Next available which I have not seen yet.