Marx's comment in the Grundrisse that labour is the 'form-giving fire'; Dipesh Chakrabarty noting Marx's vitalist metaphorics of 'living labour'; Colletti noting Lukacs quasi-vitalist thematics in relation to alienation; Gramsci's use of Bergosonian thematics of will / action filtered through Sorel; Deleuze's re-coding of Marx in terms of Bergsonian problems; Negri's quasi-vitalism of singularities of excess / biopolitics; etc. Now, I'm obviously not that happy with regarding Marxism as a vitalism, notably because of my suspicions of vitalism as a 'positive' ideological complement to capital (in fact ,its 'fit' to the value-form in implying an irreducible excess of labour always available for tapping). But it's true that there is no immediate reason to argue that Marxism isn't a form of vitalism. After all the excess of 'living labour' over any disciplining into 'abstract labour' seems to imply a vitalist thematic. The question here, I think, is one of immanence. If Marx is positing an immanent antagonism, which I think is correct, when that antagonism seems to lose effectivity then we may turn to an ontological or anthropological grounding.
Even more tricky is that Marx's arguments concerning capital's dependance on living labour for value production seems also to imply an anthropological / ontological 'surplus', in that living labour can never be fully captured. Of course, this implies an historically determined conception of the human, but, no doubt, it is not difficult to ask where this capacity comes from; is it a 'constant' or 'invariant' linked to human anthropological capacities or the 'nature of Being' (this question arose for me around Daily's Humiliation's remarks on Hallward's concept of will).
The other tendency, which I find more interesting, is what we might call an excess of the negative or an anthropology of refusal. Rather than defining the human / Being in terms of some quality of excess in and over any capitalist 'capture' here it is a matter of a strategy of refusal within the extraction / disciplining of labour. This is give a purely strategic form in Tronti, given the philosophical / ontological form of im-potentia by Agamben, and inflected anthropologically by Virno. We might call it the Bartleby option.
Again the question would be can this be given an immanent form in relation to the capital relation or does it require or imply anthropological / ontological commitments? I'm hoping the first, but the difficulty of evading the second, perhaps best thought out in Nina's generic equalitarian form of the 'human', is undeniable.