Stock on 30th June

In both case answer is same 5.4%

And what about of vat

u can take excise credit against cgst

and vat credit against sgst

Then why questions is arising that you have to pay differential amount of tax I. E (GST - vat) on the closing stock as on 30th June.
Further, it would beneficial for traders to not to sell goods on which excise +cst is levied because they will avail credit for the same in Cgst which was not available in previous act

if you are only vat holder you have stock of vat.

u will get credit of 5.4% in cgst and 5% in sgst 

whereas u will have to pay 9% cgst and 9% sgst.

which menas u got 10.4% and u will have to pay 18%

Pl refer brochure for clarification:

https://www.cbec.gov.in/resources//htdocs-cbec/gst/eflier-transition-provisions14062017.pdf