Officially the narrowest street in Vilnius is called Skapo, which is only two meters wide. However, if you ask any local to take you to the narrowest street in Vilnius, you will end up on a cobblestone street which actually is about one meter wide, has no official name and is hidden in a secret old town courtyard.
Here are the instructions on how to find it. Take Savičiaus street, where De’Žavu Vintage Boutique is, keep walking until on the right side of the street you reach M.K Čiurlionis (who was the famous Lithuanian painter and composer) memorial house and the courtyard next to it. Enter the courtyard, turn right, pass the narrow and low-pitched tunnel and on your left you will reach the narrowest street in Vilnius.
Once you are there, imagine the medieval city and neighbors shaking hands or clinking pints of beer through the windows. It is a great place to take mysterious photos.