Unified User Access would not assist in this situation, and here's why. When a client roams, an AP may have buffered traffic for the client. The APs must be able to move buffered traffic to another AP, but the original AP may not know where the client roamed to. The controller assists in this regard, and since 3rd party APs are not controlled by vWLAN, the original BSAP is not sure where to send the buffered traffic.
For your application, are you using the same SSID(s) on the Bluesocket and 3rd party equipment? If so, power settings may help. It is a client decision to roam, and clients tend to compare RSSI values to trigger a roaming event. When the RSSI value for the original AP hits a certain threshold, the client will look at the signal strength from other nearby APs. So again, power settings are always good to examine when roaming issues are present.
The minimum transmit rate can also help. Effectively, the BSAP will remove any client that cannot maintain a certain data rate. Since most clients use dynamic rate shifting to account for poor signal quality, you can be fairly certain that a client that should likely roam but has not will have a low data rate. This is another good setting to look at, however, it does not currently apply to the BSAP 1920 models.