I don't think a magic answer to your problem exists.  The accumulation of PVC on reactor internals is usually a combination of problems, related less to the mixing and more to the chemistry and process operation.
 
The most common problem with PVC accumulation is the result of refluxed monomer returning to the liquid surface and polymerizing on the vessel wall, baffles, or shaft and impellers.  Be sure that the reactor does not operate with the liquid surface at the level of an impeller.
 
The other possible solution is a slippery coating on the shaft and impellers.  Many PVC reactors are glass-lined to provide a smooth and slippery surface.  Realizing that is not a solution to your problem, other polymer coatings might be applied to the impeller and shaft surfaces to make adhesion of PVC less likely.  Polished metal surfaces also work.
 
Once polymer begins to accumulate, it seems to build continuously.

The answers by this expert are based on the best available interpretation of the information provided. The consequences of the application of this information are the responsibility of the user. If clarification is needed, please submit a further question.