attaching railing posts to deck

Hello,
I have a deck with vinyl covering. The vinyl is folded over the sides of the deck, extending down about 3.5". Tucked under the vinyl is aluminum flashing (drip edge). I plan on bolting 4"x4" cedar posts on the outside of the deck against the 2"x10" header board.
Obviously I need to cut away the bent lip of the drip edge with some snips in order for the posts to sit flush against the header board. However, I'm unsure if I need to remove all of the flashing and vinyl that extends down the header board in the areas where the posts will be attached. If I don't remove the vinyl and flashing, the cedar post may be slightly tilted because it won't be secured against a flush surface. On the other hand, I think it will be quite difficult to accurately and carefully remove just the vinyl and flashing in the areas behind where the posts will attach (cutting the vinyl with a knife might be easy, but snipping the flashing will be very hard).
Any tips or suggestions are appreciated.
thanks
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