Testing who actually opens the paid newsletter

Before I change anything about the format, I want to know if the drop-off is at the open or at the read. I'm splitting next month's four sends: two with the usual subject line style I write myself, two written by my new VA using the same template. Same content, same list, alternating weeks. I'll look at open rate and completion rate separately — I suspect they're telling me two different things and I've been treating them as one.