There are only a hand full of fights were it could make a slight difference if you exchange your vit accessoirs with str ones.
Most encounters are designed for the second tank to have a least partially something to tank.

I personally don't switch anything.
Even without Shield Oath, when equipped with Str Accessoirs, your Halone Combo tends to build up aggro fast (like really crazy fast!) to a point you have to actually stop damage - so that gear change is wasted at this point, while the damage benefit is rather underwhelming.