This is probably posted somewhere but I didn't see it in Nick's Known issues thread.
If I close Vegas 15 after using a Prores file on the timeline I find that Vegas remains in memory according to task manager. If I then reopen vegas I get a second instance of it in the task manager. Vegas appears to close but doesn't actually do so.
This doesn't seem to be the case with non-quicktime media so I assume that the internal codec is preventing Vegas from unloading. This might be a source of sluggishness for some users.
I'm running VP15 on Win7.