You can go back and retro-fit the holes in the (wooded) soffit. You'll need a zip-tool and a ladder.
You, basically, separate one vinyl soffit from the 'next' one using the zip tool, then drill your hole using a hold saw. You need a saw bit that's 3" or 4" in diameter, that's all. THen, you snap the piece of vinyl you 'dropped down' back up and move onto the next soffit.
Just do this so you can do your best in determining where to drill your first hole. Go up in the attic and measure over where the 1st and 2nd rafter are. Go outside and try to center the hole between the 2 rafters. After you drill, try to 'feel' with your hand where the rafter is. If your rafters are 16" on-center, measure over 16" (center of hole A to center of hole B), and drill. Again, feel inside the hole for a rafter. As long as you're staying inbetween 2 rafters, keep going! Maybe you need to use a coat hanger or something if you can't get your hand or fingers in there. Remember, your holes are gonna be measured ON CENTER; NOT end-to-end.