Helen,

you're comparing CS with VBAC assuming that you will have another third degree tear. You may not have a tear at all! There are techniques you can use in your labour and during pushing that will minimise any tearing and leave you feeling more empowered and in control. Many women have third degree tears and go on to have vaginal births with minimal tearing. In 8 years, I have never seen a third degree tear repeat itself.
Good luck with whatever you decide and feel free to PM me if you need.