The answer to your question about thier tactics is also explained, simply put, they appear to be winning, the vanguard seems to think that the vex are honey-dicking the cabal, drawing the cabal into a war of attrition that they may or may not lose, but that doesn't matter, because the surface of Mars is not the vexs true goal. The exclusion zone is described as "ever widening" the cabal stick with it because a. It's what they know and b. It appears to them to be working