Doesn't look to me like there was much of a swing going on; more that the Lib dem vote held steady (up a little) but the Labour vote collapsed. That could still give the GLA seat to the Lib-dems of course, but it also looks to me like Labour didn't put anything like as much energy into Vassell as the LDs - I wouldn't have thought that'd be the case for the GLA?
Not sure how the "green vote fell in Vassell" - they didn't stand last time and effectively put up a paper candidate this time.
Labour did put quite a lot of effort into it - obviously hard to tell how much but they had clearly canvassed a lot of people. I can't prove they put a lot of effort into it - but my strong impression is that they did.
Green vote - good point - you're right that they didn't stand in 2006 - I must have been thinking about when they last stood there. It was slightly more than a paper candidate though - they did make an effort to go round and deliver leaflets / talk to people a fair amount.
GLA - I suppose both parties will be putting effort fairly evenly across Southwark & Lambeth - what we're looking at here is what evidence there is from Vassall for L&S as a whole. I'd say that it shows that Labour might be in some trouble.