You will earn Reward Points for any purchases made with the Disney Visa credit card, regardless of location. However, those points will not be immediately available even if you are on-site at WDW -- points cannot be transferred to a Rewards Redemption Card until they have posted to your account, and they do not post to your account until the statement date. So unless your statement date falls during your vacation, you'll only have points to use that were earned prior to then. You will want to request your Rewards Card probably a month or 6 weeks prior to your trip, but more points can easily be transferred to it by phone or online immediately prior (or during) as long as they have posted to your account.
And as Cheshire Figment noted...it will be a less expensive rental off-site, plus you can use it to/from the parks and around your resort. Check out the FAQ sticky for off-site rental locations. If you still wish to rent at the parks, please know that they frequently run out by mid-morning so you'll want to arrive early.
Enjoy your vacation!