I see these messages often in my environment.
I ignore them.
AFAIK: VAP stands for virtual access point.
(i.e. a mac-address/wlan/SSID on a particular AP)
I assume these messages exist to alert a bandwidth analysis tool that the last set of "acsrvc_display_sta_stats()" messages for a given client came from a different AP, therefore a bandwidth calculation cannot be made based on the different between the previous and current report.