Is the scar thickness after ceserean section hat decides that in second time pregnancy we may go for vbac or not?

Sometimes. A scar thickness <4.7 mm should prompt your doctor to recommend a repeat c-section. Frequently, though, that is not the main consideration; other factors determine your likelihood to achieve vaginal delivery after a single prior c-section, such as your weight for height, prior vaginal deliveries, number of fetuses, fibroids, gestational age, labor onset, polyhydramnios, version, presentation etc.