Honestly - I would still get the vax. I had chickenpox at the age of 9 (I was in hospital with pneumonia at the time and the chickenpox showed up in a blood test).
Thinking I was covered I never bothered looking into getting the vax when I was older. I ended up with a very bad dose of the chickenpox at the age of 26. I was off work for two weeks and could barely move out of bed for the first week. I would happily go through labour and birth rather than have the chickenpox, it was that bad!!
So just from my own experience, I'd still give my boys the full MMRV vax even if they did have chickenpox before the age of 18 months.