I doubt the vitamin addition made a difference. Your temps seem fine to me. Did you dilute the rennet in distilled, cold water, and add it right away, or did you dilute and let it sit out for a while? Rennet can degrade if left sitting in water or if the water is chlorinated or has high mineral content. The rest of your steps seem OK. Maybe it was just the milk. How much CaCl2 did you add?
Sometimes, it just doesn't work like expected even if you do everything right.
5.15 is a tad low, but still above the 4.9-5.0 threshold where a tomme stops developing a pleasant nuttiness. I try to target 5.3 or 5.4 for tomme and then brine it. What was your whey drain pH when you pressed under the whey?
Tomme is basically an alpine variant, most often made with a meso culture. It grew out of a similar area as, say, an abondance. But the key differences are it's not cooked to as high of a temp, the whey drain is a touch lower to have a balance between plasticity and crumbliness, it's more moist (higher floc multiplier), and the acidity is allowed to develop after pressing to help preserve it, as there's no propionic, or if there is, it doesn't grow well because the wheel is cellared quickly and not left to ripen.
So whereas with a high-heat alpine style you target a higher end pH before brining, a tomme will tolerate a lower pH. But the key is to achieve a pre-brine pH that is a balance between acid development and calcium content (more acid = lower pH = less calcium). This point in cheese is about 5.4. It's a great tradeoff among sliceability, meltability, flavor, etc.
The commercial tommes vary significantly in terms of paste/body, so I wouldn't worry about it. Your cheese will still be good