1. Connect to the PS4 through remote play and log in on your character.
2. Press the PS button on your Vita, and select the option to go back to the Vita Live Area home screen.
3. Reconnect via remote play.
From then on your mount should work as normal, at least it does for me. After doing this and playing again using a controller, I've found the touchpad to be more sensitive and only requires an actual touch to call the map.
Just wanted to share since I originally never found a solution online