PLease do not add vermiculite to your soil. It is not natural and I think I read it can cause cancer. You would be better off buying a load of compost from your local landscape supply retailer. You can also buy "garden soil" and such at HOme Depot.
If you are worried about lead contamination your raised beds should have a bottom, and be made faily tall. Alternatively you could water frequently and shallowly so roots don't have to grow too far into the soil in search of water. This is usualy not recommended because it is kind of a waste of water, due to evaporation.

From what my previous research suggested, the danger of lead-contaminated soil has less to do with any lead that the plant absorbs, and more to do with gardening in it. Garden dort will get on your shoes and clothes, and from there into your house, where it becomes dust.

Have a dump-truck bring in cubic yards of real topsoil and compost for you, instead of messing around with potting mix.