The short i is the substitute of the final of the agni in such a dvanda samāsa in which both the members take the vṛddhi,
Thus आग्निवारुणी, which would have required long ई by the last rule: आग्निमारुतम्, which would have required आनङ् by 6.3.26. Thus आग्निवारुणीमनड्वाहीमालभेत, आग्निमारुतं कर्म क्रियेत ॥ आग्नीमरुतौ देवताऽस्य 4.2.24, takes the Taddhita affix, and by 7.3.21, both members of the compound take Vriddhi. When both members do not take Vriddhi, the rule does not apply: as आग्नेन्द्रः (आग्न इन्द्र) because इन्द्र does not take Vriddhi, by 7.3.22.
Vart:- Prohibition must be stated when the word विष्णु follows : as आग्नावैष्णवं चरुं निर्वपेत् ॥,