Here is what I would do:
1. Get that bolt down all the way while making sure the oring is put into place. If you loose all of your Freon in the process, then its just gonna happen.
2. Get your self one of those can to low port adapters:
and about 3 cans of 12 oz 134a (your vehicle holds 30 oz) and refill your system to the green part of the gauge.
The only time you need to vacuum your system is when the system is total open, allowing all freon to purge AND let air back in. This takes a little bit of time.
The worst case scenario for you, is that all your freon flows out when adjusting that bolt. Actually not all freon will flow out, because once the internal pressure reaches the same pressure as the outside (1 atmosphere) the freon will stop moving and will stay in the system. Not much but it will stay in since freon is heavier than air. The only time air will enter the system is when all freon pressure is gone and the system has been sitting "open" for a long time, or if you remove a component such as a tube, condenser, evaporator core, etc.
The reason you vacuum the system for when the system is open for a long time is that there is air in the system and air contains moisture. This will hurt the compressor as moisture is not able to be compressed. When you lower the pressure in the system by vacuuming it, it also lowers the boiling point of the moisture to that of the ambient temperature. This will boil out all the moisture and evacuate it from the system.
Being low won't hurt your system because the compressor will not kick on due to the low pressure via the low pressure switch on the lines.
When you are refilling the system your compressor won't run until there is enough pressure on the low side to initiate the compressor clutch engagement.
Most likely the pressure on the low side will be 0 or very close to it when you first start adding freon to the system. The compressor most likely won't turn on at first. Since the cans are pressurized and the pressure of the can is greater than the lines. Refridgerant will flow from the can into the system without the compressor on. Once there is enough pressure in the lines the compressor will kick on and pull from the low side to pressurize the high side. This will drop the pressure on the low side and allow more and more freon to be "sucked" into the system.