It should be vala, as a nominative.
Basic use: the grammatical subject of a sentence:
Āeksio yne ilīritas. — The Lord has smiled upon me.
Zaldrīzes buzdari iksos daor. — A dragon is not a slave.
[Where both Zaldrīzes and buzdari are nominative, although here it is difficult to tell because buzdari has the same form in the accusative. An example in the first or fourth declension would be clearer.]
This happens in some natural languages too.
And for the term nominal predicate:
A predicative nominal is a noun phrase, such as in George III is the king of England, the king of England being the predicative nominal. The subject and predicative nominal must be connected by a linking verb, also called a copula.
It's just the noun phrase which follows the copula/linking-verb to be.