Can non-brahmins who have become Srivaishnava-s perform upanayana rituals and chant Veda?

They must not. It is one thing for a non-Brahmin to be a Srivaishnava, but they cannot become a Brahmin; ‘Brahmin’ is a caste while, Srivaishnavam is a tradition. They are not permitted to wear the holy thread and perform Vedic rituals.