Although it takes Vimes to realise it, it essentially means that the local magistrates are no longer under the view of Ankh-Morpork.
So that means that the local magistrates of the Shires - if they even exist any more (I thought Vimes got rid of them?) - are now, in effect, answerable to A-M generally and to Vetinari personally.
I also thought that the trees were cut down not because of Vimes, but because Vimes's brandy-guzzling ancestor (I forget his name) ordered Hangman's Hill to be specially constructed to be so high. Or have I misunderstood?