Do you really need a pressure pot or a vacuum chamber to cast?
That would depend on alot of different factors. What are you trying to make? How much detail is involved? Will a few air bubbles bother you? How picky are you?
The vacuum chamber is used primarily to ensure a good mold. It works on the premise of vacuuming out all the air. Vacuum chambers are an expensive method. The best method, but expensive. A pressure pot works by pressuring up. You essentially compress the air molecules so that they are so small they cannot be seen. This is the method I use. I am quite picky and this method works exceedling well in my opinion.
Before I got a pressure pot, I did make some molds that were quite detailed. But that required a lot of work. I had to use a silicone with a long pot life. I would lift and drop the molds on the table from a few inches up. I would take needles and use them to poke the silicone into tight crevices. I would use my exacto knife and lightly drag it through the silicone after it was poured to break up air bubbles. And by doing all these things, I was able to make some very good non pressurized molds. But I couldnt do any real casting with clear resins. They always had air bubbles no matter what and it bugged me.
The pressure pot helps give me for the most part a very good cast. There is always 1 or 2 casts that dont turn out. Roboid was made with pressurized molds and casts. The pressure pot helps to give good clear casts and it also helps to PUSH the silicone into very tight spaces that simple gravity can't.
First and foremost, your mold is only going to be as good as the effort you put into it. Pouring the silicone is the easy part. Setting up the piece to be cast is where it makes or breaks you. Its always a pain for me to make a mold, because I want a good cast. So I take my time. I use a knife to push the clay in and around the piece to be cast. I use light brushes to clean it before making the mold. A friend gave me a halogen magnifying lamp which really helps. But it generally takes me quiet a few hours to prep a piece.
I am making clear Membros weapons for my chess set. These are the molds for them. I like to drill my own holes but decided this time not to. The Hydralaser has a small hole where the hydrapipe fits in. If you zoom in you can see that the pressure pushes the silicone into this hole. Its in the right side of the left half of the mold. Without a pressure pot, Roboid would not have been possible. The clear chest canopy was made using a pressure pot. This gave me an almost perfect mold that even replicated the threads so it could be screwed together.
Do you need a pressure pot? Again it depends upon what you are casting. But for clear resins, better molds and better casts. I would recommend using one. You dont however necessarily need a vacuum chamber. The cost/quality ratio is too high for virtually the same quality product.