Making plastic mold from plastic sheet
Depending on the material you will be casting in the mold, you may find that you can make use of a product called Hand Moldable Plastic. This product is shipped as small beads which are heated in boiling water per the instructions. I've used a hot air gun, which works faster and increases the chance of burning your fingers.

Once the white beads are heated, you can flatten and join them into sheets. Doing so will cause them to cool and become hard again. Heating the plastic once more, you can then form them around the part you wish to mold.

I've used a pasta press to form the HMP into 1 mm thick sheets. It cools quickly but also heats quickly when using a hot air gun.

A vacuum former will pull the stock material into crevices and details while this manual method may not provide the same level of detail. You can reheat the plastic as many times as needed and knead it into the model being molded, but I think some detail may be lost.

HMP is a material similar to that used in plastic milk jugs. It may release the acrylic easily, but I suggest to test it on a non-critical item to be certain.