Is there a sim card slot on the wifi PSVita?

You only need a SIM card if you've got a 3G version, and wish to subscribe to a 3G service. You do not need a SIM card at all for the wi-fi version, and if you had a 3G version but did not want a 3G subscription, you do not need a SIM card and it will function exactly like a wi-fi PSVita.