Do I really need 140 lbs of cement for a 5", 5' high vinyl fence post (31" in the ground)?
A 12&quot; hole seems overkill for a plastic fence post that can never really withstand a lot of force applied against it, at least not like a wood or steel post.
A well placed 9 or 10&quot; hole will do very well in my opinion, and reduce the amount of concrete you need.
Keeping the concrete down a few inches will allow grass to grow around the post or you can set sod &quot;plugs&quot; aside for resetting after the concrete is finished. No need to wait for it to set up, for what it's worth, no need to add water to it until the dry mix is in the hole then water it.
I typically figure 2 bags per hole when I am doing posts. either 80 or 60 lb bags. Some take more, some take less.