Final answer:
Tooth sensitivity is the most common risk associated with vital bleaching using 10% carbamide peroxide. Soft tissue reactions can occur but are less common, while superficial enamel demineralization and cytotoxicity are not direct common risks.
Step-by-step explanation:
The most common risk associated with vital bleaching using 10% carbamide peroxide in a custom tray is tooth sensitivity. Bleaching agents, like carbamide peroxide, can cause occasional sensitivity in teeth during and after treatment. This sensitivity is believed to occur due to the penetration of the peroxide through the enamel to the dentin and pulp areas, which can sometimes inflame the nerve.
Soft tissue reactions are also possible, but they're less common and typically occur due to improper use of the bleaching tray or excess bleaching gel. Superficial enamel demineralization is not a highly reported risk with 10% carbamide peroxide used under professional guidance, and cytotoxicity is a term that relates to cell damage, which is not a common direct effect of this concentration of carbamide peroxide for tooth bleaching.