Average Joe??? hey!!! What's the deal?
I think like anything, it takes practice to get good at talking to customers about this stuff. I'm not perfect at talking with customers, but I do know that I get better at it every day. It also helps to have a few visual aids at your disposal. We too have a print out that shows a zoomed in picture of a vector graphic vs raster. We also tell them that vector is important for certain applications because certain machinery requires vector line art to produce a design. And that is really the only time you need to spend on that. They don't need to fully understand it. All you really need is to convince them that having both vector and raster is better than having just raster. They can spend their own time researching the exact differences between the two, they probably won't because I'll bet you they don't really care. They just want the best option, and if you are trying to sell them original design work all you need to do is convince them that the best option is a worth while investment if they truly want to be able to use their logo or design.
Also, I think it's easier to get your point across if you have confidence when you talk about the importance of good artwork. You have to know your stuff, and believe it, or at least act like you do LOL. I had this discussion the other day with someone else at our shop who was frustrated that the customer didn't listen to them but listened to me after I was called in. And at that point I think it had to do with confidence. Everyone at our shop excels at certain things, but not everything. However, our skillets seem to compliment each other. When they were struggling to convince the customer that black lettering on their window was a bad idea, I was called into the room for support. I was introduced as an authority on the subject. Which gave me better footing going into the conversation. I said the exact same thing she did, the only difference is when I said it I put less of an emphasis on the explanation for why it was a bad idea and had more of a "this is just how it is" attitude. I was nice about it of course.
If early on you establish yourself as the expert on the subject that THEY are coming to YOU for, they tend to hear you out and trust you on all the following decisions that need to be made. You somehow need to find a way to assure them that it's in their best interest if they listen to you and that it's in your best interest if they make the right decisions, because doing so will make them more successful and if they are successful they will be able to hire you more in the future.
Really, if you think about it, you're not just selling a sign or a design. You are also selling yourself as the expert on the subject and as the go to guy or girl for these projects. Because ultimately you want them to come back for more right?