My observation is that rubber chemists add and vinyl chemists remove additives. But, this is a biased view which came after working in a company that had several rubber chemists/formulators and only one of me. I would stand in awe of their completed compounds having; accelerants, retardants, cures, antioxidants, resins, plasticizers, and many other ingredients that seemed to my untrained eye to be at counter purposes.
I’ve always thought that simple is better.
These are critical to clarity of an injection molded flexible PVC part.
Stabilizer selection - Mercapto stabilizers will work and they are relatively inexpensive. Or ask a stabilizer supplier for a stabilizer used in rear window convertables. these latter are mixed metal stabilizers. Add a little ESO, if you choose a Ca/Zn system.
Resin MW - use a low MW resin, one recommended for injection molding.
Plsaticizer - choose a plasticizer that is going to be low on water and oil extraction, low temp, and have good permanance. The mask will be wached repeatedly during its lifetime and it will be in contact w/ the user's face, a source of oils which can mar the surface. Low temp and ease of processability are also concerns. A blend is sometime better than a single plasticizer for balanced properties. What about low fogging?
Lubricants - I believe EBS, Wax OP, and Wax E are used in clear film and should work for you. Stay away from Ca. Stearate, it can cause cloudiness. Ask the guys who make products for other clear flexible PVC products to recommend a good lubircant system. Stabilizer choice and lubricant selection are critical to good clarity.
Fusion - this is critical to the clarity of any good translucent formulation. Poor fusion = cloudiness.
Stay away from TiO2 or other fillers that will cause opacity.
A bit of detail on your formulation would be of great help in troubleshooting and advising your product and answering your question. Think about the inuse of your product, before you start designing the compound or the part. Life cycle, environment of use, maintenance of long term properties are going to assure success.
Give us a hint as to where you are now; formulation, mold design, have you run trials, w/ what results, and we can be better advisors.