Create a batch file (a Notepad file that has the extension .bat) that contains this line:
tskill vixen
Then go to Start -> Control Panel -> Scheduled Tasks -> Create new task
and browse to find this batch file. Schedule it for when you want Vixen to stop.
To restart Vixen, in Scheduled Tasks, create a new task, browse to your Vixen.exe file and schedule that. You can't schedule these two events to happen at the same time -- you should schedule the batch file, for example, at 10:30 and then schedule the Vixen launch at 10:31.
HTH.
\dmc