Weak compactness criteria and convergences in $L^1_E(\mu)$

New characterizations of conditionally weakly compact (resp. relatively weakly compact) subsets in Banach space $E$ and $L^1_E(\mu)$ are presented. We discuss also several types of convergence in $L^1_E(\mu)$, in particular we generalize Szlenk's theorem on Cesàro norm-convergence of weakly null sequences in $L^1_\mathbb{R}(\mu)$ to the norm-summability with respect to a class of regular method of summability of weakly null sequences in $L^1_H(\mu)$ where $H$ is a Hilbert space.