The problem with some older houses is that attic insulation often comes into direct contact with the roof. There may be soffit vents in place, and ventilation directly above the attic insulation, but that one small area of insulation is coming in contact with the roof. The area in question is where the attic floor, the sloping roof, and the house siding all connect. That one small area becomes much warmer than the rest of the roof. It melts the snow, and as it drips further down, the roof becomes colder again, and the water turns into ice, forming the dams. If you installed two new soffit vents, that will improve overall ventilation, but it will not tackle the problem of the ice dams. What you need to do is install some baffles in that problem area to allow a smooth and complete flow of air between the soffits and the attic. This should eliminate or reduce the ice dam.