Pokemon VGC!! Need fast answers!! 100 Defenses good enough?

Well Hydreigon isn't that defensive (assuming you'll evolve the Dieno). The Dieno you're breeding seems more like a revenge killer/special sweeper. In this case, most revenge killers/special sweepers (fast sweepers in general) don't have that great of defenses, but yeah it might be able to survive, but you should expect a lot of damage to be taken.