Thank you! Setting the profile condition for the variable %SILENT to "vibrate" did the trick. Note that "vibrate" must be in all lowercase. By the way, where can I find a list of all the variables and their possible conditions?
In case it helps anyone else who's new to this, here's what I did.
1) Make a Task with the Action Alert > Notify. Title: Vibrate mode, Icon: Material > vibrate icon, check Permanent, Priority 5.
2) Make a Profile called Vibrate mode. Add State, Condition: %SILENT ~ vibrate. Add the task from 1). Long press the task, Add Exit Task. Set the Exit Task to do Alert > Notify Cancel.
I didn't need to add another condition for silent because Android still shows that icon. If that is also removed in a future update, then I will add it with the silent icon.