I'm having a section of new vinyl fencing put up.
Straight run, nothing fancy - about 50 feet. The
manufacturer is "Country Club Vinyl".

The fence company said they put their posts 3'
under grade (the posts are 6' tall above grade).
And use concrete around the posts.

Now from reading another thread on concrete and
fence posts (albeit for wood ones). Is the best
way to have them do a "quality" install, request
they -

(a). pre mix the concrete OUTSIDE the hole
(b). have them put some gravel in the hole first,
then the concrete

This installation is in NJ southern ocean county.
relatively sandy soil.

What about "frost heave" ? should there be some
specific forms to make the concrete a certain shape?
(say an upside down cone with larger end facing
down).

No contracts signed yet and I want to make sure I
spec out the best way for them to do do it.

Any suggestions/advice on "best practices" to have
it done the right way ?