In the perfect world, there is the perfect vinyl job.  Despite some
positive aspects of encapsulation, protection and buying time, an
improper vinyl job may be worse than a bad paint job.  Perservation
deems reversable techniques that does not destroy the original fabric.
Vinyl installations have cut off window sill ears, removed moldings and
treatments that have been in the way, or cladded victorian brackets that
cannot have a proper application.  And what of the frequant mildew
stains dripping from seams.  I do support proper encapsulation that may
provide temporary protection, although a long term solution it is not.
It just doesn't fit to a building's historic character.  And beware of
the salesman's justification.