It sounds like they have probably taken into consideration that the cables will pull in more than the gauge swatch they asked you to do. But if you want to assure yourself a little more, you could do like you say and make a gauge swatch using both seed stitch and the cables as they are going to be worked on the body of the vest. Then wash and block it and all that and see if the gauge you are getting is going to produce the measurements they give for the vest. Then after you get a ways into your vest measure it every little while and see if it is staying in range.
For myself, even though I do gauge swatches like I'm supposed to do, they often don't tell the whole story and I just have to decide which needle to use and go with it and make the whole project my huge gauge swatch. But most people don't seem to have as much variation in their knitting or whatever it is that causes me such trouble though.
I want to be able to check to make sure I'm on track before it's too late
I know what you mean, you want to avoid having to start over, but with me, I often have to start over. It is never too late, just more work. :-)