(Long time lurker, first time poster!)
I had a similar experience this season with Vardaman sweet potatoes, but due to staggered plantings was able to make a couple of additional observations. I planted two beds of Vardaman in late May, and a single bed about three weeks later in mid June. When I harvested the May planted beds last weekend, 95% of the of the roots had cracked. The majority of those that were not cracked were from plants located in what I would expect to be the driest part of the bed (south side next to the edge of the bed). When I harvested the June planted bed, 95% of the roots had NOT cracked.
My area received a ton of rain just around the time I believe the skins would have been hardening up for the May planted beds, and since then almost no rain (during the time the June planting would have been hardening). I think this supports what others have posted here regarding watering/rain being the culprit. Even with very good drainage conditions (20" high concrete block raised beds with 6" stone footers, amended with compost and mulched) the May planted beds still split. This leads me to believe that there’s not much you can do to prevent damage in the event of ill-timed heavy rains, other than maybe physically covering the beds.
While we’re on the topic, here are a few other observations regarding Vardaman in raised beds. Planted at 16"x13" spacing, 9 slips per 4’0 x 3’4 bed, the May beds yielded 2.0 lbs/plant. The June bed yield 1.1 lbs/plant, but was planted in a bed that received significantly less sun than the other two. The vines in the June bed were noticeably shorter than the other two and the roots themselves were thinner. The vines in the May planted beds ranged from 3’ to 5’ long, which is a very manageable size for raised beds. And lastly, and maybe most importantly, they keep well and taste great! Equal to and often better than any supermarket variety we’ve purchased.