While time is the central category in Marx's thought, unevenness is the core of Lenin's.
For Lenin, all real-world processes develop according to their own contrasting temporalities. At a particularly large scale, we're used to thinking of "uneven development" as a factor which distinguishes societies from one another; e.g., Third World countries from First. For Lenin, unevenness is fundamental to all processes of change. All of these things develop at different rates: societies; the classes within societies; the layers, fractions and groupings within classes; the individuals within different layers.
Lenin's notion of the "vanguard" is a corollary to this central concept of unevenness. The "vanguard" of a class is that portion of the class which has developed furthest at any moment in time. A "vanguard party" tries to orient toward these leading layers, as opposed to the more backward elements.
Thus for Lenin the vanguard continually shifts. Elements or individuals who are ahead of the others one day may fall behind, potentially overnight. This is always inevitably true: it's a consequence of uneven development.
Stalinist practice devalues unevenness. Declaring that party and vanguard are equivalent amounts to claiming that the party is outside of time.
Comparable devaluation of unevenness is one of the hallmarks of contemporary left sectarianism. While declaring themselves to be the vanguard, sectarians in practice orient toward the greenest, most backward elements of the milieux in which they operate. As you participate in antiwar coalitions, watch the sectarian cadre at work: who do they try to recruit? Always, the inexperienced.