“Vivent les rois” is the grammatically correct form. But “Vive les rois” is more commonly used, and is accepted I think. The expression come originally from “Que vivent les rois”, litteraly “Let the kings live”, the french expression being the equivalent of “Long live the king(s)”, but its shorter form “Vive les rois” is now more understood as “Hooray for the kings”, hence the singular “Vive” instead of the plural “Vivent” for the verb “Vivre” (to live).