Hi guys, I found this topic http://forum.mikrotik.com/t/rc9-performance/16899/1
it seems related to my problem, with vap enabled overall tx ccq drops below 40%, all clients connected to normal interface may disconnect;
disabling VAP, tx ccq rises above 90% ( wireless card R52H - mode G ).
only 1 user connected to VAP/Hotspot and 16 users connected to WLAN in PPPoE mode.
Regards to all
Alessandro