It's an interesting thing that the eruption of Vesuvius actually did less damage than it could of. It is thought that 6 pyroclastic surges occured, the first 2 striking Herculaneum and Oplontis, 3 and 4 striking Pompeii, and the remaining 2 heading further out to bury Stabiae and the intervening area.
However, a lot depended on complex factors which could easily have changed on the day. So, I shall start with a simple change of wind direction. About hafway through the first day of the eruption, the wind direction begins to change, swinging around from the NE to the NW. This has some key factors. Those attempting to escape from Stabiae by boat will now be able to as the wind is no longer pushing their sails back onto the beach. The ash cloud will begine to move, eventually settling into a position which spreads pumice over the other side of Bay of Naples, possibly as far as Misenum. The rescue ships of the Imperial Navy will be scattered earlier due to the earlier threat, though butterflies may alow for the rescue of Rectina from Herculaneum. Pliny will also be blown back to Misenum (again possibly with Rectina) but may subcumb to asthma aggreavated by the ash there as it is speculated may have happened at Stabiae.
The biggest effects, however, will be the pyroclastic surges. The first 4 continue as OTL, buring Herculaneum, Oplontis and Pompeii. The last two, however, strike Neapolis and possibly Puteoli.
With the population of the largest city on the bay dead, and the Imperial fleet in need of repairs, what is the effect on the reign of Titus and beyond?