Brothers, oh brothers, my days here are done, the Dornishman's taken my life, But what does it matter, for all men must die, and I've tasted the Dornishman's wife!
Following the provided grammatical rules, this should translate to "The singing knight" as there is no issa denoting is.
It doesn't. High Valyrian does not form a present continuous tense in the way English does (to be singing, he is singing etc.), instead it uses the present tense as in this sentence. "The singing knight" would be Vāedare azantys.
Issa would be used as a copula, usually to define something. Singing is a verb, not a noun or adjective.