Other considerations

One of the things that I've done is with the playdays still adhered to the currently accepted definition of what a vintage kart is- which is 1975 and before. I've always thought this was a weird and arbitrary date- I mean there was no reveloutionary change in 1976- I have always thought either 1972 or 79 made more sense. But I did this largely because I knew there were guys in my area who had the right idea (if they had a 1973 kart, it was built to 1973 specs and equipment) and had been attending all the meets I had either helped promote or attended and I didn't want to shut them out. Your group might be different. If there are 8 of you and you only have rear engine karts, that might be your opportunity to do rear engine only playdays. It's up to you. Remember, the object of the playdays is to accommodate a need that already exists, not to promote a meet try to reach a critical mass, entice people to come and create a demand for your meet. With this "we're a group of folks who just want to go play" attitude/philosophy, that's what allows you to have an amazing day at the track with only 4 or 5 other people.  
I will however, make the arguement about keeping things in 1975/76 as the upper limit of the range. At some point, the late 70's/early 80's karts (post-vintage, to borrow a phrase from the vintage motocross world) start to handle and behave like the modern karts. We've already established it's a bad idea to mix modern with vintage karts on the track. So running with post-vintage, it becomes just like running with modern karts, which is asking for trouble. There is even a grey area if a early to mid-70's sidewinder is running sticky tires instead of carlisles, or vintage speeds, or chen shins.
If you want to accommodate these folks, I have two possible suggestions, both of which I have tried:
I. Contact the host club and explain the situation and see if they will allow "post-vintage" karts to run with the modern karts. This last time out the club agreed, and I was up front about if you bring a Yamaha, you can run- but it will be with the modern guys. No one brought a post-vintage kart.
II. Ask those guys to organize themselves and see if you can sell the club on splitting the sessions three ways(vintage, post-vintage and modern) with the allure of bringing in more revenue for the club. Probably a hard sell at the first couple of playdays you do, but if you get established and are successful and build a good relationship with the track, it might work. In my case, I never really heard back from the couple of guys who wanted to run post vintage karts. I put them in touch with each other and laid out the possibilities and let them take it from there- and I don't think anyone has.
If something like that does take off, you might then be able to roll back one group to 1972, and have post vintage as 1973-83, or whatever they want to do. That's pushing the boundaries of the simplicity of the setup and starting to get dangerously close to becoming a bit of a hassle, which is at odds with the whole deal. But if you are presented with that situation it might be a solution.
The final thought I have is that I've always felt the key to the playday deal being successful is that it is two-fold: it facilitates the need to have a low key, non-race platform for folks who care about keeping the vintage aspect pretty pure; and in fulfilling that need it provides something for the club that accommodates you. To me it's all about building relationships- scratching each other's back, if you will.
This is just strictly my opinon and I could be proven wrong, but I tend to want to reward those relationships with the track/club. I know there are some karters who would like to see each playday at a different track, just to have the excitement of running someplace new all the time. I understand that, but I lean towards trying to build an ongoing relationship with one or two tracks so you can grow the playdays as a team. It's gotten to where each of the two clubs I deal with really roll out the red carpet for us, because they know they can count on us once or twice a year. Something a little different from their regular schedule, yet we're not bugging them to be out there all the time. As they get more familiar with you, you become friends with the club's board- and they are more accommodating. If you are in an area of the country where there is potential to tap into more karters who are on the same page, you can provide them with increased revenue for their good faith. I feel very fortunate that I have the kind of relationship with Kinsmen Kart club in Dixon, CA and Blue Max Kart Club in Davis, CA that on my end, a playday usually just takes one phone call to set up. But it took doing a few playdays to establish a relationship in order to get it to run that smoothly. I'm not sure that would have happened had we moved around all the time. Just something to think about.
Best of luck in your playday endeavors- prepare to have a ball!