Press the PS Button + Select to bring up the RetroArch menu, then toggle the "On-screen Overlay" to use the keyboard.
The Vita is a powerhouse for its age, but DOS emulation is demanding. Based on the latest community testing: 286-Era Games (100% Speed) : Classics like Wolfenstein 3D run flawlessly at full speed. 386-Era Games (~70% Speed) : Games like Corridor 7 ps vita dosbox new