My original answer (below) was correctly pointed out by beofett not to answer the OP's question. I made a different logical consstruction, which I stand by, and was critical of the lack of decision making in leaving the child not vaccinated, but I missed the actual question. So, I will answer it directly.
Question: vaccinate or deliberately expose?
For generations we have intentionally exposed our children to chickenpox because it is generally harmless to children and much worse (and dangerous) to adults. So we effectively immunized the kids by giving it to them, sparing them a potentially nasty case of it as an adult.
Now that there is a vaccine, we can do the same thing without either the child or the parents actually having to suffer through a bout of the disease. This is great, because chicken pox just plain sucks. The vaccine was not available until after my daughter caught it at preschool.
So the question is .. are the risks and downsides of getting the disease worse than the risks and downsides of the vaccine. The medical profession uniformly think so, although the cynics contend that they are in the hip pocket of big pharma. I say, if you don't trust your doctor, find another one.
So I repeat my answer .. vaccinate. The risks are negligible (see the wikipedia excerpt below), and both you and the child are spared a terrible couple of weeks.
From the wikipedia article on the vaccine...
The vaccine is exceedingly safe: approximately 5% of children who receive the vaccine develop a fever or rash, but as of 1 May 2006, there have been no deaths yet attributable to the vaccine despite more than 40 million doses being administered. Cases of vaccine-related chickenpox have been reported in patients with a weakened immune system, but no deaths.
If your child is below the vaccination age (~ 1 year I think), the decision is easy. Don't expose now, but vaccinate at the appropriate age.
So let's answer the question assuming your child is older than 18 months or so.
If you are asking the question, then you have already made the vaccine decision because your child is not vaccinated. I disagree with that decision, but that ship has sailed.
Given that, the decision to expose or not expose is binary. The vaccine issue shouldn't enter into it. Are you prepared for your child to get chicken pox now, and would you prefer a sure case now to a likely case at some unknown time in the future. Paraphrasing Harry Callahan, do you feel lucky?
Me, I expose if I can make the arrangements, primarily because when Murphy hits me, he hits me hard.