Gas Porosity Problem

I am producing custom personalized sterling silver jewelry with resin printed models. I am using plasticast as investment. My prodicton is very low at one time. I am casting maximum five rings so maximum silver weight about 100g ( including sprues and button ). I am using electrical furnace and vacuum for casting. Firstly I tried to cast without adding air vents to our models but there was very big problem in my castings. Then I add vents to my models which is connected to top of flask. There was 5 ring models but three of them is good others have gas porosity. ( There are many little sphericel pits on the ring). Another thing that silver in the air vent did not reach same level with button. My opinion is that, liquid silver was solidyfied very quickly due to low volume of silver and gas trapped.
My flask temparute 500 °C degree. I am thinking to increase about 600 °C.
What would you suggest.

If you are vacuum casting do not add air vents leading from surface of flask to mold. Have vents run from ring to bottom or side of flask. If this does not work then suggest you up your temp and maybe examine your casting metal temp

Thank you for your reply. What must be vents open to outdoor or close ? I added to open to outdoor. Another thing that is there a possibilty of issuficient vacuum ? I am thinking also add a big tank about 20 liters. I will make pre-vacuum before starting of casting.