Quick and Dirty
This is one of those situations where the DM has to make a ruling on the fly.
Me, I'd rule that an instantaneous effect like fireball will clear away all the small stuff like grass and ivy, while damaging but not destroying dense undergrowth and mature trees.
Doing it properly
If you want to slow the game down and work out the thickness of all the plants in the area the rules say that Wood has a hardness of 5 and 10 hp per inch of thickness. Remember that energy attacks (sure as fire) do half damage. Plants are not specified, but I would assume no hardness and 3 hp per inch of thickness. Since most plants used by entangle are less than 2 inches thick, its a non issue really.
Unattended objects also take this damage.
The fireball sets fire to combustibles and damages objects in the area. It can melt metals with low melting points, such as lead, gold, copper, silver, and bronze.