I think this is best answered in two parts:

Getting to and from Reims in one day:

There is now a TGV (high speed trains) train that runs between Paris and Reims- it takes 45 minutes (I believe this is new in the past couple years- used to be only the slow trains.) The slower trains take 2 hours for the journey. So depending on which trains you take, getting to and from will take 2 to 4 hours in total, just in train journey time.

Once you’re in the Champagne region, it’s easiest explored by car, IMHO. This way you get to go at your own pace, stop at some of the smaller vineyards where tours might not stop and get different treatment than you would if you were with a big bus tour.

You can get great free Champagne driving route maps (there are 4 different laid our routes, if I remember correctly.) I’m sure there are lots of ways to hire someone to take you around- from personal tours to bus tours. I’d suggest calling the tourist office in Champage or doing a quick google search on the topic as I can’t give you personal advice on this.