score:20
You are writing fiction - there is no need to be 100% accurate. The Vienna sewer system was, and still is, regarded as one of the most impressive in Europe. Although repairs from WW2 bombing were not completed until 1950, this did not interfere with a fascinating portrayal of the sewer system in 1949's The Third Man.
My opinion is that if the protagonists encounter, and need to detour around, bombing damage once or twice, your audience will willingly suspend any disbelief that the portrayal is 100% accurate.
Note: Bombing damage was sufficiently severe that there were legitimate cholera concerns, as the system had become essentially non-operational. This in essence means that there was significant damage somewhere on every major drainage route in at least one place. However no cholera epidemic occurred (suggesting that seasonal flooding was never extreme in the 5 years immediately post-war, though I was unable to confirm this), and complete restoration in just five years suggests that the total extent of damage was moderate. Serious bombing of Vienna only started in 1944 after the Western Allies established an airfield at Foggia in Italy. The most severe bombing was not until February and March of 1945.