When will the Draytek Vigor 3900 be able to Centrally Manage the AP-900's ?

I doubt it will..only the 2860 and 2925 can do it.
We have a couple of site now you use a 2860 purely for Wi-Fi management

Thats a pity , got 1 location ( Guesthouse ) with a Vigor 3900, currently deployed AP-700 x 4 in the building , wanted to expand to some AP-900's

This is in fw version 1.3 although I my 3900s cannot see my AP900s/