The install begins at the ribbon board, just above the foundation. It should be straight and level from there. IF the house looks 'crooked' by the time the vinyl gets to the roofline, either the vinyl is not level OR the house is crooked.
You didn't say if the 'problem' is on the side of the house or on the front. If it's on the side, it's probably not too bad since no one is really 'looking' up there. HOWEVER, if it's on the curb-side, then EVERYONE will see it.
If it's on the curb side, and w/o knowing why the vinyl is crooked, I can't say what you should do. For starter, I'd leave the vinyl straight. If you install it to 'match' the roofline, the house will DEFINITELY look crooked! And later, when and if you ever decide to replace your roofing shingles, perhaps you can talk to the Roofing Contractor about your options.
Other than that, I don't know what else to say. My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!
That is the best idea assuming that you are going to reside anyway.