In Vedic Sanskrit, the devī and vṛkīs inflections are two types of inflection of feminine ī-stems exhibiting distinct apophony patterns.
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Vṛkīs
The distinguishing feature of the vṛkīs inflection is that the ī always has the Vedic accent except in the vocative case, and the nominative singular has the desinence -s like non-feminine words. Indeed, while vṛkīs-words are overwhelmingly of the feminine gender, there are a few members of the class that belong to the masculine gender or are gender indeterminate: rathī- "wain-driver, charioteer" (often applied to Agni, who trafficks sacrificial offerings and divine boons between mortals and immortals).
The inflectional type is usually accepted to reach back into Proto-Indo-European times, with an exact correspondence of Sanskrit vṛkīs and Old Norse ylgr, both meaning "she-wolf", first described by Karl Verner in 1877 (see Verner's law). The distinction between devī and vṛkīs dies out in during the Vedic period and Pāṇini is unaware of it, classifying ī-stems by accentuation (vṛkīs-words are a subset of NīS).
One formation that has been diachronically connected with the vṛkīs inflection is Cvi, which in Pāṇini's grammar of Classical Sanskrit refers to a formation where an ī is added to a nominal stem and compounded with a verbal root kṛ "to make", as "to be" or bhū "to become", resulting in a factitive verb where the ī-stem is indeclinable and used like a preverb. For example, grāmībhū "to get possession of a village", from grāma "village".
Devī
The devī inflection exhibits an ablaut pattern different from the vṛkīs inflection. Pāṇini does not make the distinction, classifying the ī-stems by their accentuation (devī words may be NiiN, NiiP or NiiS).