I really need help here! How long can I apply for a C-visa after I apply for a D-visa? Do I have to wait for some time(for example 180 days)? Do I violate any rules if I stay more than 180 days based on a D-visa and a C-visa altogether?? Or I have to make sure my stay within one year is less than 180 days?
Any time you spend in a country for which you have a valid D visa does not count for the purposes of the 90/180 rule. You should therefore be able to apply immediately. However, your application is likely to receive greater scrutiny, so you shouldn't plan to return to Austria unless you have a very clear reason to motivate your trip and a clear and compelling reason to return to your country of residence after the end of your trip.