You are eligible to take credit of eligible duties in respect of inputs held in stock on the appointed day subject to following conditions:-
a) the said goods are eligible for input tax credit under GST Act too.
b) you are in posession of invoice and other prescribed documents evidencing payment of duty under the existing law(here in your case VAT).
c)such invoices or prescribed documents were issued not earlier than tweleve months immediately preceeding the appointed day.
IF your asked credit satisfies above all conditions you can take credit of entire amount.
Ther are some basic conditions required to avail ITC in respect of transition of credits like,you must have filed all the returns required under the existing law for the period of six months immediately preceeding the appointed day and others.For these refer sec 140 of the CGST Act.