Vakyapadiya 2.5

2.5

saṃbodhanapadaṃ yac ca tat kriyāyā viśeṣakam
tathā tiṅantaṃ tatrāhus tiṅantasya viśeṣakam //

And the vocative word is a modifier of the action; / similarly, they say a word ending in a finite verbal suffix modifies a word ending in a finite verbal suffix in that context.