So far, there isn't anything really worth buying a Vita for. Sure, the incoming Persona 4: The Golden is tempting, but too little to buy a handheld. Shin Megami Tensei 4 was announced too, but still too little and too late.
When these games come out, the Vita will be probably cheaper, so no need to buy the Vita now. And even then, I'll only consider buying it if more interesting games are announced.
The 300€ would be probably better spent on a PC upgrade for the Watch_Dogs, Grand Theft Auto 5 and ArmA 3.