I am not an expert on Tin Whiskers, but everything I have read or been told by experts is that there is no method to 100% guarantee that Tin Whiskers will not develop. There is detailed information on the iNEMI and many other web sites.

Vacuum would help but will not prevent it.

The only way to prevent whisker growth under any condition is to remove the driving force, which is the compressive stress, from the Sn coating. This can be done quite effectively by using the thin Sn (<3 um

When there is a large external applied (regardless intentional or non-intentional) compressive stress, it could induce whisker growth in the Sn layer also - it is a matter of time.

Unfortunately, your terminology is slightly confusing. Tin whiskers can penetrate paralyene. On the other hand, if the interface is weak, it may seek a more preferential path under the paralyene.

If the interface is weak, vacuum impregnation would likely have minimal effect unless it places the paralyene under stress. But, you then risk damage to your components.