Can you ever stop the raiders?
Solution 1:
Well I couldn't stop them too with the vault door. It's just like the fire. It will appear some time and will appear ever. :-D
Only way is to get your hardest boys with the strongest armor and the biggest guns into your vault door to kick their asses as fast as they can.
By the way, upgrading the vault door decrease the chance that a raid will occur.
Solution 2:
Upgrading the vault door to advanced and beyond won't stop raiders from breaking in, but it will give you more time to get your best armed and armored dwellers to the vault to keep them at bay, which can be important if those dwellers are assigned to rooms very deep in the mountain.