1. Call the manufacturer of the vent that you are installing and ask them. (I bet there is no instruction manual included).
2. Ask the home owner what they would prefer.
I vote for asking the home owner before you install them, what they would like before you install them, (and write it down and have them sign it), because, right or wrong, if they are not happy, YOU won't be happy!
This could be a 50/50 chance of getting paid!
Best wishes, Jason
ps.. It came to me that you are talking about the long rectangular type of vent, not the round ones I see so often. Again, with vinyl, this is not an issue!