1) Vector 4 armies to a city
2) From the city that has 4 armies vectored to it, vector from that city to another city.
3) Now try to cancel your vector from the city that has 4 armies vectored to it (ie click on the city with 4 armies vectored to it, click on the vector button and click on your city with 4 armies vectored to it to cancel the vector).
The game reports you already have 4 armies vectored to this city.
You have to first undo 1 vector to the city then you can cancel your vector.
KGB