I'm glad you're using vinyl - they'll not be bothered by termites or
dry rot. I did a small picket fence project in my yard last year. I
dug holes and sunk the posts about 12". . .I used Quikrete, pouring it
dry from the bags into the holes. I had put a few rocks around the
posts to level them and hold them in place. Then just took the garden
hose and turned on the water.
My posts were wood that I treated with that "greenstuff" to keep the
pests out (for awhile). I made certain the Quickrete level ended up
being level with the ground.
With your using vinyl - you will likely have a more durable outcome.