Based on what you say it sounds like the bug is that it only opens the port when the emulation starts. So if the configuration changes then it would have no effect, particularly if the port was no configured at all at startup.
Probably needs to (try) close and reopen the serial device after any serial configuration change. The complicating factor would be if VT tries to leave the device closed when not in use. Then it could be opening and closing the port based on some other heuristics as well. I helped with the Linux serial code but I have not looked at it in a very long time. -- John.
