On implementation of GST if your turnover exceed 20 lakhs you have to apply within 30 days in form GST REG 01. A whole process of verification and approval will follow and within 3 days you will be granted a registration certificate. You can register voluntarily also. All this you can do after implementation of GST.
New registration under GST are yet to be started. So it is advisable to wait for registration or else you believe that threshold limit under state vat law will get exceeded before appointed date, then take registration under vat and based upon provisional I’d take GST registration.