Depending on which HiveManager platform you are using, the guides below review how to link your Aerohive network to an existing Radius server. Are you trying to funnel all authentication requests to that external server through one AP, or are you only using the one AP? As for the VLAN, you would just set that in the user profile of the Radius SSID and you should be good to go there.
HiveManager Classic-https://thehivecommunity.aerohive.com/s/article/External-Radius-Server
HiveManager (follow the first section, and then focus on the instructions for creating an external Radius server)- https://thehivecommunity.aerohive.com/s/article/Radius-SSID-in-NG
This guide will walk you through how to determine which HiveManager platform you are using, for reference: https://thehivecommunity.aerohive.com/s/article/Which-HiveManager-Platform-am-I-using
