I'm in the Midwest, and I agree with Ride7359. I see RM1(SS)'s point, but I don't think it applies in this case.I don't think that what RM1(SS) says is necessarily the case. I thought of that and then disregarded it as too narrow. If you have been on vacation for a week (or two or three) as planned and you are extending your vacation, I still think the "more" adds emphasis even if it is the first time you are extending it.
It may be a usage issue. Sometime the Midwest and the East Coast speak slightly different languages!