how high was mount vesuvius (mount somma?) before it erupted in AD 79?
The AD 79 eruption is the last of four caldera forming events which reduced the heigth of Monte Somma. Their approximate timing is: ca. 18000 BP, ca. 8000 BP, ca. 3400 BP and AD 79. 

Between 3400 BP and AD 79 it was about 1300 m high and height of the intact stratovulcano before the beginning of the caldera collapse was about 1900 m.
Note that while the original height can be extrapolated from the shape of the cone, the remaining height after individual events is not well constrained.

Reference: Cioni, R., Santacroce, R., &amp; Sbrana, A. (1999). Pyroclastic deposits as a guide for reconstructing the multi-stage evolution of the Somma-Vesuvius Caldera. Bulletin of Volcanology, 61(4), 207-222.