I do the vegetable garden in clay soil, without problems. Fertilizers and manure will acidify the soil, enough for garden. Some vegetables prefer clay, most are neutral (if there are enough nutrients). If you can, add some sand and good soil on top. One problem of clay soil is that it tend to dry quickly (but less than sand soil), and it could be hard (so forget carrots).
Tannin are taken from trees, so it is natural. I doesn't think we (as human) we get problems, but it is also a good herbicide, so not so nice for vegetables. The choice of soil is much more important (heavy metals).
In general I recommend raw wood. You need to replace from time to time, but this is good: it gives you the possibility to improve the bed (construction), and rot wood will give back some nutrients.
I confused tannin with tanalised procedure. Tannin were used (and still used) to preserve wood and leather. Tanalised wood has much more chemicals. I doesn't think enough to get health problems (still oligo-elements, so essential for us, and it should not give so much to vegetables, and then we eat just a part (fruits or some leaves).