I have been testing the latest Beta for the Vacuum 1.6.126. But this tip should work with any version of FW being used.
I have discovered the following helps to get the Vacuum back on track. When you notice the Vacuum appearing confused and drawing phantom walls and room, simply Pause the cleaning (from the app), Pick up the Vacuum (Yes, pick it up), go to another area of the room (I went to an opposite corner), place the Vacuum down, and then select Resume cleaning (From the app). This caused the Vacuum to acclimate to the room again and get back on track. I had to do this twice, but the entire room got cleaned.