First, do prapatti. These days many castes have mixed with the Vishvakarma clan. If the Vishvakarma is a Vaishya (for whom rituals like upanayanam are applicable), the grandson can perform the last rites. If not, if the wife attains Paramapadam first, the husband can perform the last rites; if the husband attains Paramapadam first, a close male relative or another male Brahmin can accept the kaippul from the wife to perform the last rites. For the one who attains Paramapadam last, a close relative or another male Brahmin can do the rites.