Most tent floors aren't fully waterproof. They're decent but if you stand on a puddle, water will probably be forced through and dampen the inside. So adding a footprint helps mitigate that issue (along with damage protection) and extending the foot print reintroduces the issue by wicking moisture in.
Also the point of a vestibule is usually so that when you enter/exit a tent in rain, none of the rain gets into the tent body (lots of poorly designed vestibules unfortunately). So if you add a foot print, when you enter/exit it will trap rain and now your gear which you wanted to protect is floating in a puddle.
I would suggest just using a cheap/light polycro groundsheet. If it gets wet it's easy enough to shake off, it's light, versatile, and it'll be good for keeping dirt off your gear without much of the hassle that extending a groundsheet comes with.