[@Willy Vereb]

The way I'd see it, it was originally accepted by the king at the time of the marriage but was marketed more as an equal alliance to the people (they need not know after all) and afterward the relationship become more and more ambiguous. Thus, the king is aware that he is... theoretically, a vassal of the emperor, but he doesn't tell anyone about that detail and when the Emperor calls to war, he answers... but markets it as doing so as an ally to his own people. If confronted about what his allegiance is without room to wiggle away in a non-committing answer, he'd probably say he's not a vassal (Unless he's really desperate for help on that moment).

I'd say the previous king could have been awarded this honor in a scenario such as: Another Vinculi nation waged war on a then smaller elvish empire and had decent chances to win. Seeing its chance, the king of the previous nation decides to say he is utterly convinced on the evish envoy's request for help and will come to aid against the unjust Vinculi invader with all his might! The aim of course is to snag territories. This doesn't go as planned as the king gets trashed in the Vinculi mainland, but this allows for the elves to claim a victory on their homefront. As a consolation prize, the Emperor offers the title as well as a daughter to the king. He normally wouldn't have accepted, but his choice to go to war left him in a bad posture and, with his armies defeated, he suspected others might try to wage war against him. So he accepts, to gain diplomatic weight against his hostile neighbors by threatening them with the weight of a superior ally. Hell, I'd say the two nations could have fought a few wars together.