Zone 4 - curious about grafting tomatoes and overwintering to harvest slips for transplant

Newbie to growing in northern Wisconsin - 2019 will be our first season and only for personal use that year. We'd like to get to a marketing point eventually. Initially planning tomatoes in grow bags with our own soil and compost and probable "hoop" protection. Of course, we want heirlooms and will choose short-season, indeterminate varieties and hope to save seed. I'm wondering if VFN are a problem in a cold climate? If so, what do you all think about grafting heirlooms to hybrid/VFN resistant root-stock? I understand the process of grafting, but do NOT understand how it works to improve the plant! I know that a tomato is actually a perennial, and that I can save/overwinter it indoors or a greenhouse long enough to take slips/clones for replacement. But I am confused as to whether that clone will have the disease-resistance ... AND if seeds from that grafted plant will have resistance.
I'm in awe of all of you working so hard on breeding and researching - thank you so much for your efforts and sharing of your knowledge and seeds!! I'm looking forward to hearing your expert advice on my newbie questions and wishing the best results to us all.

Hi Mary Beth, most people around me get early blight.  Late blight occasionally comes this far north with certain weather patterns but it's rare.  I don't know about V, F and N.  Maybe check with your local extension people to see if they're a problem up here, I hope they'd know.