If I connect my PSVita to the DGL-5500 using WPA or WPA2, with AES encryption the Vita will try to connect to the PS4, then say the connection has dropped immediately after connecting. (Meanwhile, all traffic on the WAN side gets dropped for a few seconds, this can be confirmed with ping -t www.google.com, then the traffic comes back with 1,000-3,000ms ping times for a few more seconds before it goes back down to 15-20ms.)
If I connect my PSVita to the DGL-5500 using TKIP encryption the Vita will successfully connect to the PS4. Every 4-5 minutes though the router will spike to 1,000-3,000ms ping times and the Vita will glitch out as if it's lost connectivity.
If I hook up another WiFi AP, and connect the Vita to that one everything goes fine, no ping spikes or WAN packet losses. I'd prefer to not have to keep another (older) device hooked up solely for WiFi.
The same exact behavior happens with my Google Chromecast - another commonality between the Vita and the Chromecast is Streamboost interprets both as 'FreeBSD' on the list of devices page.
This is using v1.11B03 firmware.