That'd be a strange interpretation of what it actually says, which is that the king must swear an oath to "God and the holy king Olaf". While swearing an oath of fealty to a king could be interpreted as making yourself a vassal, it's fairly clear from the context that Saint Olaf is being invoked in his spiritual role as patron saint of Norway rather than for his worldly role as king. (c.f. the Swedish 14th century law of Magnus Eriksson, which requires an oath to God, Saint Mary and Saint Erik. And while Erik was venerated as a saint-king, it's not certain whether he even ruled all of Sweden. Virtually nothing is known about him outside of what his family wrote when they were trying to have him canonized; and that's all focused on legitimizing him as a saint and does not have much in the way of trustworthy facts about his rule)

Anyway, the law was replaced with Håkon Håkonsson's 1260 law of succession and again in 1273 with Magnus Håkonsson's law, and many more since. Had it remained in force it'd likely have disappeared with the Reformation, when veneration of saints decidedly went out of fashion. The Norwegian Constitution of 1814 did establish that coronations are to be held in Nidaros Cathedral where Olaf is buried, though. (Stortinget abolished coronations in 1908, but there's still a blessing ceremony for new kings there)