There is no actual reserve fleet any more. Due to relentless cost presure, the fleet has been squeezed more and more over the past 20 years or so to the point that VBZ has hardly any spare cars to play with.
As Robin says, part of the response is that there will no longer be training runs at peak times. Also, there will no longer be private hire at peak times. But even so, the fleet will be pretty vulnearble if several cars have to be taken out of service due to accidents or unscheduled repairs.
There was a time VBZ were considering borrowing some of the surplus BLT trams, but this was finally rejected due to the modifications and overhauls that would be required.
In the extreme bus substitution cannot be ruled out on the lighter routes such as the 15.
On the trolleybus routes bus subsititution has sadly become a normal thing as there too the fleet has been honed down to a level that even if all available trolleybuses are in service, the peak demand cannot quite be met.
Only in the case of the tram fleet it is to be assumed that this will be rectified as the new trams enter service.