OK, first of all your pressures are quite low. Here's the problem: at 57°F ambient the saturated pressure of R-134a is only 53.6psi. You are showing about 103psi on the high side, which corresponds to a condensing temp of only 89°F. If you try to add refrigerant to bring your pressures up, you will end up with a system that is severely overcharged when the weather is warm.
Here's the other issue you have at low ambient temps: expansion valves (and orifice tubes) have a finite maximum capacity. Thus, the maximum amount of refrigerant that can flow through them is determined by the pressure differential across them. A typical automotive TXV has a rating of 1.5-2 tons (18000-24000btu/h), but that rating assumes a pressure difference of at least 150psi across the valve. In other words, if your low side is 30psi, which is an evaporating temp of 34.5°F, you would need a high side pressure of 180psi in order to ensure the evaporator is fed enough refrigerant. These are just approximate numbers - actual values depend on the particular valve, but those figures are in the ballpark.
Now, your system is running a ΔP (delta P, aka pressure difference) across the valve of only 80psi. This means your valve has only about 53% of it's nominal capacity, perhaps less. Your evaporator is going to be starved of refrigerant. If you're using a cheap Chinese valve then it could be starving the evaporator even more. The line coming out of the valve will still be frosty cold, but what little liquid is being admitted into the condenser will evaporate away before it makes much of a dent in the temp of the air flowing through it. In fact, since your vent temp is higher than ambient it sounds like your getting heat from your heater core leaking into the air stream.
What is the rated capacity of R-12 for your system? I'd take that value, multiply by 0.8 to find the 80% value, and charge your system with that amount of R-134a. Then, with the system running, slowly lower a piece of cardboard over the front of your condenser to partially restrict airflow. Watch your gauges carefully while you're doing this. Shoot for a target of 225-250psi on the high side. When you get there, rig the cardboard to stay put (watch your coolant temp!!!), and then let the system stabilize for a few minutes. Once it has, note your high and low side pressures and measure your center vent temp. Assuming your TXV (expansion valve) is functioning correctly, then once you have your high side pressure in that 225-250psi range your low side pressure and vent temps should be on the normal to lowish side, depending on how much heat leakage you have from the heater or engine bay.
