I do think the second verse diverges from the main topic that is most likely about Kurt and Courtney. Some orange juices, especially ones that are not 100% freshly squeezed orange juice, use vitamin D3 as a fortification. D2 comes from Plants, while D3 comes from animals. So Chris is musing most likely sarcastically, about whether he can still be a fan of the band, if he is not a true vegan / vegetarian.
I do think the second verse diverges from the main topic that is most likely about Kurt and Courtney. Some orange juices, especially ones that are not 100% freshly squeezed orange juice, use vitamin D3 as a fortification. D2 comes from Plants, while D3 comes from animals. So Chris is musing most likely sarcastically, about whether he can still be a fan of the band, if he is not a true vegan / vegetarian.