Is the current best approximation ratio for Vertex Cover problem also a lower bound?
Cormen, Leiserson, Rivest and Stein say that this algorithm that achieves ratio of 2 is tight. They did not exclude the possibility of another algorithm achieving better.

Vertex Cover is NP-hard to approximate with a factor better than 1.36:
http://annals.math.princeton.edu/wp-content/uploads/annals-v162-n1-p08.pdf

Also check the following paper which give a 2-epsilon inapproximability result based on unique games hardness:
http://www.sciencedirect.com/science/article/pii/S0022000007000864

For approximation algorithms, see the wiki article for Vertex Cover and in particular the paper of Karakostas 2009: https://eccc.weizmann.ac.il/report/2004/084/